Merge githubsucks/main into gpu-initial-target

Integrate folding Stage 2 and its landed-state documentation with the
protocol-v20 GPU initial-target branch. Preserve per-session fold projection
selection in the target bootstrap transaction and retain v19 compatibility
coverage after the later protocol bump.

docs/active-work.md

@@ -1,6 +1,6 @@
 # Active work — cross-machine resume ledger
 
-**Snapshot: 2026-07-23.** This file records volatile work that has not
+**Snapshot: 2026-07-24.** This file records volatile work that has not
 landed on `main`. Read it after `docs/agent-handoff.md`. Remove completed
 entries when their PR merges; do not let this become a second permanent
 backlog.
@@ -14,9 +14,10 @@ backlog.
   machine-local: `origin` may name this canonical URL, a release mirror,
   or something else, and therefore has no authority by name alone.
 - Canonical base at this snapshot:
-  `githubsucks/main` @ `47581f4` (web grammars #146 atop folding Stage 1
+  `githubsucks/main` @ `b168dca` (folding Stage 2 landed-doc refresh #150
-  #142, inline-math framing #145, and one-command GPU invocation #141;
+  atop folding Stage 2 #149, the ledger refresh #147, web grammars HTML+CSS
-  protocol v19).
+  #146, and the LaTeX Stage 1 #144 / inline-math framing #145 pair; protocol
+  v19).
 - On the transfer source, `origin/main` named a release mirror at
   `d3fa632` and lagged badly. On the current destination, `origin` names
   the canonical URL. This difference is why all recovery begins by
@@ -50,7 +51,7 @@ git worktree list
 git status --short --branch
 ```
 
-The `git log` command must expose `47581f4` or a newer intentional main.
+The `git log` command must expose `b168dca` or a newer intentional main.
 If it does not, stop and repair the remote/fetch configuration.
 
 ## Active lane: GPU initial target
@@ -61,21 +62,24 @@ If it does not, stop and repair the remote/fetch configuration.
 - Approved framing branch: `githubsucks/gpu-initial-target-framing`;
   Revision 2 checkpoint `71039d1`.
 - Original implementation base: canonical `githubsucks/main` @ `c49a8c7`
-  (folding Stage 1 #142); current canonical `main` @ `47581f4` is integrated
+  (folding Stage 1 #142); current canonical `main` @ `b168dca` is integrated
-  conflict-free by merge `d6d4be6`. Protocol was v19 before this work.
+  before publication of the second-review merge. Protocol was v19 before this
+  work.
 - State: implementation checkpoint `2dd30ec`; first review-fix checkpoint
-  `bef1c08`; second-review checkpoint `be8c67c`. Integrated, smoke-tested,
+  `bef1c08`; second-review checkpoint `be8c67c`. Current `main` is integrated,
-  fully gated, and published at protocol v20. PR #148 remains open for user
+  smoke-tested, and fully gated at protocol v20; merge publication is next.
-  review: `https://github.com/levineuwirth/pmacs/pull/148`.
+  PR #148 remains open for user review:
+  `https://github.com/levineuwirth/pmacs/pull/148`.
 - Scope delivered: one session-scoped `pmacs --gpu [--socket …] FILE` target,
   protocol-v20 semantic bootstrap, launcher-owned tilde/cwd resolution, exact
   Unix path transport, pre-window target readiness, replica coherence, and the
   approved behavioral acceptance matrix.
-- Second-review verification: formatting and strict Clippy; 1,801 default +
+- Post-integration verification: formatting and strict Clippy; 1,815 default +
-  1,978 CRDT library tests; target gate 1 default + 14 CRDT; M4 121; required
+  1,992 CRDT library tests; target and invocation gates 14 CRDT each; Folding
-  GPU 152; Vterm Stage 3 5 default + 7 CRDT; isolated-config workspace sweep
+  Stage 2 48 CRDT; M4 121; required GPU 152; Vterm Stage 3 5 default + 7 CRDT;
-  3,272 across 87 suites. The prior two-window Wayland/Vulkan isolation smoke
+  isolated-config workspace sweep 3,334 across 88 suites. The prior two-window
-  remains valid; this round changes only daemon failure/publication behavior.
+  Wayland/Vulkan isolation smoke remains valid; this round changes only daemon
+  failure/publication behavior.
 - Deferred unchanged: automatic GUI selection, multiple files, general
   live-open commands, packaging, and remote GPU paths.
 
@@ -88,41 +92,25 @@ git worktree add --track \
   githubsucks/gpu-initial-target
 ```
 
-## Folding framing lane (Arc 6)
+## Folding lane (Arc 6) — Stages 1 and 2 MERGED; Stage 3 (GPU) is next
 
-- Portable branch: `githubsucks/folding`; worktree `../pmacs-folding`.
+Both shipped stages are on `main`; nothing in this arc is in flight. Stage 3
-- Base: **rebased onto canonical `main` @ `96d0bae`** at implementation
+has **no branch and no framing yet**.
-  start (was `cac4961`; the earlier base fell behind the docs + tab-width
-  housekeeping).
-- Framing head: revision 5 of `docs/folding-framing.md` (rev 1 → … → rev 4
-  absorbed three review rounds; rev 5 records approval + the Q#FD4 binding
-  decision).
-- State: **Stage 1 (fold engine, headless) implemented; PR #142 OPEN**,
-  two review rounds landed on the branch. Bindings decided (Q#FD4 → Emacs
-  hideshow `C-c @` set); Bet B1 accepted as framed.
-  Load-bearing decision (Q#FD1): the bundled grammars ship no fold query and
-  no `folds.scm`, so the roadmap's "tree-sitter fold ranges" is not free; v1
-  is structural node folding (block-like node ≥2 source lines, derived head
-  line, closer-aware tail), with indentation fallback and curated queries
-  deferred. `FoldState` already exists in the protocol; Stage 1 starts
-  *producing* it (authoritative-empty), no protocol bump; gutter markers are
-  frontend-derived like the diagnostic sign bars, so no new wire type. Staged
-  like vterm: Stage 1 engine (headless), Stage 2 TUI, Stage 3 GPU.
-- PR: **#142** (`Arc 6 folding — Stage 1: instance fold engine`), open
-  against `main`. Round 1 (tail-boundary delete bug + buffer-kill cleanup +
-  close-all point-move) and round 2 (pin the kill-path + close-all through
-  the real command surface) are landed as fix commits on the branch.
-- Next: land Stage 1; Stages 2/3 are separate branches/PRs, each re-framed
-  in detail after the prior stage lands.
 
-Recovery worktree:
+- Stage 1 (headless fold engine) merged as **#142**, Stage 2 (grid/daemon
-
+  collapse) as **#149** — both under "Closed since the last snapshot".
-```sh
+- Retained, carrying nothing unmerged: branches `folding` / `folding-tui`
-git worktree add --track \
+  and worktrees `../pmacs-folding` / `../pmacs-folding-tui`. The framings
-  -b folding \
+  `docs/folding-framing.md` (rev 5) and `docs/folding-stage2-framing.md`
-  ../pmacs-folding \
+  (rev 4) are the approved artifacts Stage 3 re-scouts against.
-  githubsucks/folding
+- **Stage 3 (GPU) obligations, already named by the framings** — the
-```
+  starting point for its own framing doc: GPU collapse at TUI parity;
+  caret/hit-test fold-awareness; the `BufferSnapshot` **fold-mirror clear**
+  (parent R2-4 — without it, empty-after-revert diff suppression leaves
+  stale folds on the GPU, the same trap class as #120); CRDT-origin and
+  GPU-optimistic interactive unfold (parent R2-3); and flipping
+  `FrontendView.fold_projection` to `true` for semantic frontends, which
+  Stage 2 deliberately left `false` (Q#FD21).
 
 ## Parked lane: kill-ring browser + persistence
 
@@ -160,6 +148,72 @@ git worktree add --track \
 
 ## Closed since the last snapshot
 
+- **Folding Stage 2 (grid/daemon collapse) — MERGED as #149** (`main` @
+  `6ed4fe9`, 2026-07-24, after **five** review rounds). The grid TUI now
+  renders collapses. Spine (Q#FD12): `src/fold_view.rs`'s `VisibleLineMap`,
+  derived from the fold store plus a window's line offsets and **never
+  stored**, threaded as `Option<&'a VisibleLineMap>` on a lifetime-bearing
+  `Viewport<'a>` that stays `Copy`. No wire schema or protocol change; the
+  GPU path is Stage 3. 48 acceptance tests on the real `paint_frame` grid,
+  every behavioral claim bite-verified. Durable design points, each a trap
+  Stage 3 inherits:
+  - the map's unit is a **merged hidden component** (overlapping *or
+    adjacent* intervals unioned, keeping the earliest visible head), not a
+    fold — folds may cross, and a later fold's own head can be hidden;
+  - instances are **per rendered window** and **per command/event
+    operation**, never per frame; a command's map follows the operation's
+    **target** window, since a wheel event names a pane without activating it;
+  - fold projection is **per-frontend** (`FrontendView.fold_projection`) —
+    shared `EditorCore` motion would otherwise make a simultaneous unfolded
+    GPU session's cursor skip lines it still displays;
+  - a hidden cursor normalizes by **position**, not row, and `set_view_top`
+    clamps in the setter rather than being repaired at render time;
+  - the interactive-Lua unfold keys on the **post-intercept** edit site — a
+    managed buffer intercept may legally relocate the op.
+
+  Process notes worth keeping: `main` moved under the arc, and the merge was
+  textually clean but **not semantically clean** (#146 added `Viewport`
+  literals the new `folds` field invalidated) — a clean `git merge-tree` does
+  not mean the merged tree compiles. CI was red at review on the macOS/luajit
+  `outline_5_level_100_entry_renders_within_100ms` budget flake and went
+  green on rerun.
+
+- **Documentation ledger refresh — MERGED as #147** (`main` @ `0a479ae`,
+  2026-07-24). The #142 housekeeping, expanded after review found the ledger
+  stale through four merges rather than one. Its own macOS/luajit red was the
+  vterm `VTERM_ALT_READY` PTY timeout; green on rerun.
+
+- **Web grammars HTML + CSS — MERGED as #146** (`main` @ `47581f4`,
+  2026-07-23). `.html/.htm/.xhtml` and `.css` highlight off the official
+  `tree-sitter-html` 0.23 / `tree-sitter-css` 0.25 crate query constants (no
+  in-repo overlay), and HTML's `INJECTIONS_QUERY` lights up `<script>` → js
+  and `<style>` → css. Durable lesson recorded in
+  `docs/web-grammars-html-css-framing.md`: the `highlight.rs` capture table
+  is **global**, so adding a capture name retro-paints every other language —
+  check the reverse direction and pin it.
+
+- **LaTeX Stage 1 — MERGED as #144**, with its parent inline-math framing
+  committed as **#145** (`main` @ `f09b0a1`, 2026-07-23).
+  `.tex/.latex/.sty/.cls` highlight via `codebook-tree-sitter-latex` 0.6 plus
+  the first in-repo query overlay (`builtin/queries/latex/highlights.scm`,
+  `include_str!`) — the reusable pattern for grammars whose crate ships no
+  usable queries. The crates.io `tree-sitter-latex` is provably broken (no
+  `scanner.c`). The math parser and Tiers 3–4 are deferred to the inline-math
+  arc.
+
+- **Folding Stage 1 (headless fold engine) — MERGED as #142** (`main` @
+  `c49a8c7`, 2026-07-23, after three review rounds; round 3 clean). The
+  instance-side fold store + translating/dropping `View`, the structural
+  source (derived head line, closer-aware tail), the Lua data API +
+  interactive `C-c @` commands, the command-path pre-edit unfold, and
+  authoritative-empty `FoldState` production landed with no protocol bump.
+  The `folding` branch and worktree (`../pmacs-folding`) are retained but
+  carry nothing unmerged; the `folding-framing.md` framing is preserved.
+  CI red at merge was an unrelated environmental perf flake
+  (`outline_5_level_100_entry_renders_within_100ms`, macOS/luajit only),
+  green on rerun. Stage 2 has since merged as **#149** (above); durable
+  substrate seams live in `docs/agent-handoff.md` §1.
+
 - **Vterm Stage 3 (protocol v19 + GPU terminal) — MERGED as #135** (`main`
   @ `cac4961`, 2026-07-22, after two review rounds). Arc 5's terminal stage
   is complete (compile mode #113, Stage 1 #126, Stage 2 #130, Stage 3 #135).

docs/agent-handoff.md

@@ -1,12 +1,13 @@
 # Agent handoff — cross-machine continuity
 
-**Last updated: 2026-07-24, after GPU initial-target PR #148 second-review fixes
+**Last updated: 2026-07-24, after GPU initial-target PR #148 second-review
-and verification completed on branch `gpu-initial-target` (protocol v20),
+fixes were integrated with folding Stage 2 (#149) and its landed-doc refresh
-following one-command GPU invocation (#141), the documentation refresh (#140),
+(#150), following web grammars HTML+CSS (#146), the LaTeX Stage 1 / inline-math
-Vterm Stage 3 (#135), tab-width rendering parity (#137), locals-query
+framing pair (#144/#145), folding Stage 1 (#142), one-command GPU invocation
-processing (#134), modeline detection (#132), mode system wiring (#129),
+(#141), the documentation refresh (#140), Vterm Stage 3 (#135), tab-width
-config registry (#127), Vterm Stages 1–2 (#126/#130), and completed Themes
+rendering parity (#137), locals-query processing (#134), modeline detection
-Arc 4 (#120/#124/#125).**
+(#132), mode system wiring (#129), config registry (#127), Vterm Stages 1–2
+(#126/#130), and completed Themes Arc 4 (#120/#124/#125).**
 This file is the
 bridge between development machines. If you are an agent reading
 this on a fresh clone: this document plus the `docs/*-framing.md`
@@ -20,9 +21,11 @@ commands, read `docs/active-work.md` immediately after this file.
 
 ## 1. Where the project stands (2026-07-24)
 
-- `main` @ `47581f4` (web grammars #146 atop folding Stage 1 #142,
+- `main` @ `b168dca` (folding Stage 2 landed-doc refresh #150 atop folding
-  inline-math framing #145, and one-command GPU invocation #141), protocol
+  Stage 2 #149, ledger refresh #147, web grammars #146, LaTeX Stage 1 #144 /
-  **v19** (`SUPPORTED=[6..=19]`; v19 = terminal frames/events).
+  inline-math framing #145, and folding Stage 1 #142), protocol **v19**
+  (`SUPPORTED=[6..=19]`; v16 = `ThemeFacts`, v17 = `FontFacts`, v18 =
+  `StatuslineSegments`, v19 = terminal frames/events).
 - **GPU INITIAL TARGET IMPLEMENTED — PR #148 under user review**
   (`docs/gpu-initial-target-framing.md` rev 3; branch `gpu-initial-target`).
   `pmacs --gpu [--socket NAME|PATH] FILE` now transports exact Unix path bytes
@@ -41,6 +44,71 @@ commands, read `docs/active-work.md` immediately after this file.
   Existing no-target managed launch, direct attach, TUI, and legacy protocol
   behavior remain intact. See `docs/active-work.md` for the portable checkpoint
   and verification.
+- **Folding Stage 1 (headless fold engine) LANDED — #142**
+  (`docs/folding-framing.md` rev 5; merge `c49a8c7`; three review rounds,
+  round 3 clean). Arc 6's engine — instance-side and headless; **no frontend
+  renders a collapse yet** (that is Stage 2). No protocol bump.
+  - `src/fold.rs`: a per-buffer `FoldStore` of byte ranges attached as a
+    translating/dropping `View` (the `BufferStyleSpanTranslator` pattern — it
+    translates strictly-inside edits and DROPS boundary-crossers,
+    provenance-blind); `FoldRegistry`/`SharedFoldRegistry` =
+    `Rc<RefCell<HashMap<BufferId, {Arc<Mutex<FoldStore>>, ViewId}>>>` (the
+    SyntaxRegistry per-buffer model), held on both `EditorCore`
+    (`src/editor_core.rs:223`) and `EditorState` (`src/editor.rs:110`).
+    Containment is **start-exclusive, end-inclusive `(start, end]`**; the
+    stored range is `[end of head line, end of last hidden line]` (NOT the
+    `ByteRange` struct doc's `[start,end)`).
+  - Structural source: nearest enclosing block-like node ≥2 source lines →
+    resolve introducer↔body → **derived head line** (the line immediately
+    above the first hidden line, so wrapped signatures / `where` clauses stay
+    visible) → **closer-aware tail** (a closing-delimiter line stays visible,
+    e.g. `} else {`). Stale/absent parse tree refuses.
+  - The six `EditorCore` edit primitives call `unfold_before_point_edit`
+    first (command-path pre-edit unfold, keyed on the active frontend's
+    point). Interactive Lua-command unfold (yank/query-replace/comment) is a
+    Stage 2 obligation; CRDT-origin unfold is Stage 3.
+  - `src/lua_bindings/fold.rs` (`install_fold`): `pmacs.fold.*` data API
+    (explicit buffer, no ambient resolution, matching #127) + interactive
+    commands on the **Emacs hideshow `C-c @` prefix set**;
+    `builtin/runtime/fold.lua`.
+  - `src/semantic_render.rs`: `fold_state_msg` PRODUCES `FoldState`
+    (semantic/GPU sessions) — authoritative-empty, diff-suppressed, per-session
+    baseline reset on `BufferSnapshot` (the #120 stale-mirror trap class; the
+    GPU fold-mirror clear-on-snapshot is a named Stage 3 obligation).
+  - Durable lesson (round 2): after wiring a cleanup into a production hook,
+    PIN IT THROUGH THE REAL PATH — a direct-call unit test misses the wiring
+    (falsify by revert).
+  - **Stage 2 (grid/daemon collapse) LANDED — #149** (merge `6ed4fe9`; five
+    review rounds; `docs/folding-stage2-framing.md` rev 4). Its
+    load-bearing reframe: the TUI had **no non-identity
+    source-line↔display-row map** (`view_top + row` was baked into ~13 sites),
+    so Stage 2's spine is `src/fold_view.rs`'s `VisibleLineMap` — derived from
+    the fold store plus a window's line offsets, never stored — that the render
+    loop, gutter, diagnostics, caret, selection, peer presence,
+    viewport/scroll/motion, and the mode-line indicator all route through, plus
+    the interactive-Lua unfold widening. Threaded as `Option<&'a
+    VisibleLineMap>` on a lifetime-bearing `Viewport<'a>` that stays `Copy`.
+    Design points the review rounds forced, each a trap for Stage 3:
+    - the map's unit is a **merged hidden component** (overlapping *or
+      adjacent* intervals unioned, keeping the earliest visible head), not a
+      fold — folds may cross, and an inner/later fold's own head can be hidden;
+    - instances are **per rendered window** and **per command/event
+      operation**, never per frame; a command's map follows the operation's
+      TARGET window (a wheel event names a pane without activating it);
+    - fold projection is **per-frontend** (`FrontendView.fold_projection`, set
+      at attach from the negotiated `semantic_render` bit) — shared
+      `EditorCore` motion would otherwise make a simultaneous unfolded GPU
+      session's cursor skip lines it still displays;
+    - a hidden cursor normalizes by **position**, not row, and `set_view_top`
+      clamps in the setter rather than being repaired at render time;
+    - the interactive-Lua unfold keys on the **post-intercept** edit site — a
+      managed buffer intercept may legally relocate the op.
+    No protocol bump. **Stage 3 (GPU) is next and has no framing yet**; its
+    named obligations are GPU collapse at TUI parity, caret/hit-test
+    fold-awareness, the `BufferSnapshot` **fold-mirror clear** (parent R2-4 —
+    the #120 trap class), CRDT-origin / GPU-optimistic interactive unfold
+    (parent R2-3), and flipping `FrontendView.fold_projection` to `true` for
+    semantic frontends.
 - **One-command GPU invocation LANDED — #141**
   (`docs/gpu-invocation-framing.md` rev 6; merge `63fbc66`; two implementation
   reviews). The additive public path is `pmacs --gpu [--socket NAME|PATH]`;
@@ -440,8 +508,16 @@ commands, read `docs/active-work.md` immediately after this file.
     lexical resolution, settled per-layer facts, shared TUI/GPU
     local-predicate filtering, and a registry-wide locals-query invariant
     shipped without a protocol change.
-  - Remaining ranked arcs: 6 folding, 7 DAP, 8 GPU splits, plus the
+  - **Arc 6 (folding) Stages 1 and 2 LANDED — #142 and #149** — the headless
-    `.ipynb` arc (its JSON-grammar prerequisite shipped in #123).
+    fold engine (store, structural source, Lua `C-c @` surface, command-path
+    unfold, `FoldState` production), then the grid/daemon collapse (the
+    `VisibleLineMap` spine, fold-aware gutter/diagnostics/caret/selection/
+    presence/viewport/motion, and the interactive unfold widening). **Stage 3
+    (GPU) is next**, unframed.
+  - **Web grammars HTML+CSS LANDED — #146**, and **LaTeX Stage 1 — #144**
+    with its inline-math parent framing **#145**.
+  - Remaining ranked arcs: 6 folding Stage 3, 7 DAP, 8 GPU splits, plus
+    the `.ipynb` arc (its JSON-grammar prerequisite shipped in #123).
 
 ## 2. How we work (the part that must not drift)
 

docs/folding-stage2-framing.md

@@ -0,0 +1,728 @@
+# Folding Stage 2 — grid (daemon-rendered) collapse — framing (Arc 6)
+
+**Revision 4 — 2026-07-23. Status: DRAFT for review (rounds 1–3 findings
+addressed).** Parent architecture (`docs/folding-framing.md`, rev 5) is APPROVED
+and Stage 1 (the headless fold engine) is MERGED as **#142** (canonical `main` @
+`c49a8c7`). This doc reframes **Stage 2** in detail off that base, per the
+parent's §8/§14. Numbering continues the parent's `Q#FD…` scheme from `Q#FD12`.
+
+## 0. Revision history
+
+### Round 1 (rev 1 → rev 2)
+
+- **F1 (major) — nested folds resolved to the wrong head.** Rev 1's `head_of`
+  returned the *innermost* enclosing fold's head and assumed it visible; under
+  nesting an outer fold hides the inner head, so carets/diagnostics/relative
+  numbers would have clamped onto another *hidden* line. Rev 2 defines
+  **`visible_head_of(line)` = the head of the *outermost* enclosing fold** (the
+  only head guaranteed visible), used by every clamp. A hidden `view_top` clamps
+  **backward** to that head (not forward past the fold, which contradicted
+  acceptance 6). Relative/Hybrid numbering anchors on the **clamped visible
+  cursor**. New nested-fold / shared-cursor acceptance (§11).
+- **F2 (major) — the mapping-consumer census was incomplete.** Rev 1 claimed all
+  painters route through `Viewport`. They do not: local selection
+  (`src/editor.rs:3241`) and the mode-line scroll indicator (`src/editor.rs:3803`)
+  paint from `paint_frame` with raw `view_top` arithmetic; peer cursor/selection
+  (`src/overlay_paint.rs:159`) runs *after* `paint_frame` and subtracts `view_top`
+  independently; the style overlay (`src/overlay.rs:385`), search wash
+  (`src/search.rs:492`), diagnostics (`src/diag.rs:563`), and the completion
+  anchor each derive `start_line + row_offset`. Rev 2 carries the **complete
+  consumer census** (§3.1), adds **TUI peer-presence fold behavior** (clamp a
+  hidden peer cursor to the visible head; drop/project hidden selection cells) as
+  explicit scope, and expands acceptance to pin local selection, peer presence, an
+  ordinary style/search overlay across a fold, completion anchoring, and the
+  scroll indicator.
+- **F3 (major) — the fold glyph's sign cell does not exist by default.** Line
+  numbers default to `Off` ⇒ `gutter_width()==0` (`src/window.rs:216`); diagnostics
+  then fall back to a col-0 *background* on the first content cell
+  (`src/diag.rs:635`), so rev 1's "zero-width-change" gutter glyph had nowhere to
+  render. Rev 2 makes the fold glyph **conditional on a gutter existing**
+  (Q#FD20): gutter off ⇒ **ellipsis only**; gutter on ⇒ reuse the sign cell with
+  diagnostic priority. Acceptance covers both. The parent's unconditional
+  gutter-marker promise, if required, needs a **dedicated sign column** and an
+  acknowledged layout change (named alternative, §5/§9).
+- **F4 (major) — a per-frame map instance cannot serve command-time motion, and
+  `Viewport` is `Copy`.** `move_up/down`, paging, wheel scroll, and clicks execute
+  **outside** the render frame, so they cannot reuse a map installed on that
+  frame's `Viewport`; and `Viewport` is compile-time-pinned `Copy`
+  (`src/view.rs:130`). Rev 2 reframes the map as **one shared derivation/query
+  primitive** with **separate short-lived instances** for rendering vs
+  command/event handling (Q#FD12); the render instance is threaded as
+  `Option<&'a VisibleLineMap>` on a **lifetime-bearing `Viewport<'a>`** (a shared
+  ref is `Copy`, so `Viewport` stays `Copy`); the primitive's home is usable from
+  `EditorCore`, not render-only.
+- **F5 (moderate) — tighter unfold seam + settled undo decision.** Rev 2 keys the
+  Lua-path widening on the existing **`InteractiveCommandOrigin`**
+  (`src/editor.rs:53`), not command-history inference; hooks the **common
+  `run_buffer_edit`** (`src/lua_bindings/mod.rs:1305`) so interactive
+  `bypass_intercept` mutations do not escape; **requires the target to be the
+  invoking frontend's active-window buffer** (an explicit inactive-buffer Lua
+  mutation stays programmatic); keeps the Rust hook at `apply_active_edit` and the
+  `notify_buffer_edit` exclusion; and **explicitly defers undo/redo unfold**
+  (Q#FD19).
+
+### Rulings absorbed
+
+- **Q#FD17 = INCLUDE.** Vertical motion steps through *visible* lines; if motion
+  begins from a hidden logical cursor (shared fold or goto-line), it **first
+  normalizes to the visible head**, then steps to the preceding/following visible
+  line (§7).
+- **#142 housekeeping** (retire the active-work.md folding lane + refresh handoff
+  §1) is a **separate docs PR**, kept out of `folding-tui` (PR #147, ready for
+  merge but not yet landed).
+
+### Round 2 (rev 2 → rev 3)
+
+- **F1 (major) — fold-aware motion must be frontend-projection scoped.** Q#FD17/18
+  changed shared `EditorCore::move_up/down/page_*`, but Stage 2 leaves the GPU
+  visually unfolded (Stage 3). With a TUI and a semantic/GPU session on the shared
+  fold store simultaneously (`src/daemon.rs:876` — a frontend holds exactly one of
+  a grid `RenderState` or a semantic `SemanticRenderState`, both may attach to one
+  buffer), visible-line motion would make the **GPU cursor skip source lines it
+  still displays**. Rev 3 adds a **per-frontend `fold_projection_active`** decision
+  on `FrontendView` (`src/editor_core.rs:240`), set at attach
+  (`register_frontend_view`, `:540`) / cleared at detach (`:549`): grid/TUI ⇒ true
+  (Stage 2), semantic/GPU ⇒ false until Stage 3. **All command/event-time
+  visible-line reckoning** (motion, paging, wheel, click inverse, auto-scroll) is
+  gated on the acting frontend's flag; render-time clamps are already grid-path-only
+  (a semantic session never enters `paint_frame`). New simultaneous TUI+semantic
+  Down/Up/paging acceptance (Q#FD21, §7, §11).
+- **F2 (major) — render maps must be per window, not per frame.** `paint_frame`
+  renders several windows that may show **different buffers**
+  (`src/editor.rs:2922`, `reg.get(window.buffer_id)`), and the presence pass
+  iterates recipient windows by `buffer_id` (`src/overlay_paint.rs:124`). Rev 2's
+  "builds one instance" was wrong. Rev 3 specifies **one map per nonterminal
+  rendered window**, keyed on that window's `buffer_id` + its `TextView`
+  line-offsets + `view_top`; peer presence derives/receives the **recipient
+  window's** map (§3.2). New acceptance: a split with two **different** buffers,
+  only one folded — no active-buffer/map leakage (§11 acceptance 14).
+- **F3 (moderate) — hidden positions need column projection, not only row
+  clamping.** Rev 2 clamped a hidden caret/peer cursor to `visible_head_of`'s row
+  but left the column unspecified, which could paint a hidden point at an arbitrary
+  column on the head. Stage 1's precedent is exact: folding moves point to the
+  fold's `ByteRange.start` (the end of the visible head line). Rev 3 adds
+  **`visible_position_of(pos) -> Position`** mapping a hidden byte to the
+  **outermost** enclosing fold's `range.start`, used for local/peer carets and
+  selection **endpoints** (hidden selection interiors stay dropped). New acceptance
+  with a hidden cursor whose column differs materially from the head's end (§3.1,
+  §7, §11 acceptance 6).
+- **Nits.** Fixed the `Viewport<'a>` typo; stated the map build cost honestly as
+  O(folds) with a byte→line lookup per fold (B4).
+
+### Round 3 (rev 3 → rev 4)
+
+- **F1 (major) — command-time maps follow the operation's target window.** Rev 3
+  said every command/event map came from the active window, but wheel events call
+  `scroll_window(win_id, …)` for the pane under the pointer **without activating
+  it** (`src/editor.rs:1919/2264`). In a split, scrolling inactive buffer B could
+  therefore derive folded buffer A's map. Rev 4 separates the two axes: the
+  **acting frontend** supplies the Q#FD21 projection-policy gate; the
+  **operation's target window** supplies `buffer_id` / line offsets / `view_top`
+  for the map. Motion, paging, and auto-scroll target the active window; click
+  inverse and wheel target their explicit `win_id`. Acceptance 14 now pins an
+  inactive-pane wheel with different fold state.
+- **F2 (moderate) — byte projection must handle crossing overlaps, not only strict
+  nesting.** Stage 1's `FoldStore::insert` permits arbitrary normalized ranges and
+  rejects no crossing overlap. With A hiding lines 1–3 and B headed on line 2 and
+  hiding 3–5, a point on line 5 is directly inside only B; B's `range.start` is
+  nevertheless hidden by A. Rev 3's "outermost fold containing the original
+  point" could therefore return a hidden position. Rev 4 makes the derived map's
+  unit a **merged hidden component** retaining its one visible `head_line` and
+  exact `head_position`; both `visible_head_of` and `visible_position_of` resolve
+  through that component. This is equivalent to repeatedly projecting a hidden
+  fold head until visible and covers nesting, shared heads, and crossing overlap.
+  Acceptance 6 adds the crossing shape.
+- **F3 (minor) — PR #147 is ready, not landed.** Corrected the two premature
+  "landed" claims; the housekeeping remains separate and unmerged.
+- **Nit — selected render kind, not an advertisement.** Q#FD21 now keys on
+  `session_state.negotiated_capabilities.semantic_render`, the same attach-time
+  boolean that selects `RenderState` vs `SemanticRenderState`; LOCAL is explicitly
+  grid-projecting.
+
+## 1. What Stage 2 ships
+
+The grid TUI is **daemon-rendered**: the daemon walks a buffer's source lines and
+paints a character grid it ships to the terminal client. Stage 1 built the
+instance-side fold store and produces `FoldState` for GPU sessions, but **no
+frontend renders a collapse yet** — the daemon grid path never consults the fold
+store.
+
+Stage 2 makes the **daemon grid renderer fold-aware**:
+
+1. **Collapse** — omit each fold's hidden source lines; head line shows a trailing
+   ellipsis; rows below shift up.
+2. **Gutter fold marker** — a fold glyph on the head-line row **when a gutter
+   exists** (Q#FD20); ellipsis-only otherwise.
+3. **Fold-aware line numbers** — the `LineNumbers` family skips hidden lines;
+   relative/hybrid distance is measured across the collapse, anchored on the
+   clamped visible cursor.
+4. **Fold-aware diagnostic signs** — a sign on a hidden line clamps to the fold's
+   **visible head** row (most-severe merge).
+5. **Fold-aware caret, local selection, and peer presence** — no caret, no
+   selection cell, and no peer cursor renders on a hidden line; each clamps to the
+   visible head or is projected/dropped.
+6. **Fold-aware viewport/scroll/motion** — on **fold-projecting (grid) frontends**,
+   `view_top`, paging, wheel scroll, clicks, auto-scroll, vertical line-motion, and
+   the mode-line scroll indicator all reckon in **visible** lines; a semantic/GPU
+   session on the same buffer keeps raw-line motion until Stage 3 (Q#FD21).
+7. **Interactive-Lua-command unfold widening** — yank, query-replace, and
+   comment-toggle unfold a fold at their edit point before the edit is visible.
+
+**No wire schema changes and no protocol bump.** `FoldState` production (Stage 1)
+is untouched; the TUI collapse is entirely daemon-side (the vterm-Stage-2 shape);
+the GPU render path is Stage 3.
+
+## 2. Ground truth (scouted + review-verified 2026-07-23, `main` @ `c49a8c7`)
+
+### 2.1 The grid render path is layered — collapse belongs deep in it
+
+`RenderState::render_frame` (`src/instance_render.rs:98`) is a **diff shell** (it
+double-buffers cells, emits `CellDelta`); it does not walk source lines. The
+source-line→grid-row loop is two calls down:
+
+```
+RenderState::render_frame  (src/instance_render.rs:98)   — diff shell
+  → paint_frame            (src/editor.rs:2796)          — per-window composition; HAS state.fold_registry
+      → window.text_view.render(buf, viewport, grid)     (src/editor.rs:2948)
+          → TextView::render (src/text_view.rs:207)      — the line→row loop
+```
+
+`TextView::render` (`src/text_view.rs:213`) maps `line = start_line + row_offset`
+— **strictly identity**, no skip, no wrap (long lines truncate). `Window.view_top`
+(`src/window.rs:179`) is a **source-line index**. `DisplayCoord`'s doc
+(`src/view.rs:108`) anticipates a non-identity map "once virtual lines, wrapping,
+and inline expansions appear," but nothing implements one today — **folding is the
+first**.
+
+### 2.2 The complete set of identity-assuming consumers (F2)
+
+Every site below assumes `display_row = source_line − view_top` (or the inverse)
+and must consult the shared visible-line map. Rendering-frame sites, after-frame
+sites, and command-time sites are distinguished because they need **different
+instances** of the map (§3, F4):
+
+| # | Site | file:line | phase |
+|---|---|---|---|
+| 1 | text render loop | `src/text_view.rs:214` | render (Viewport) |
+| 2 | line-number gutter | `src/editor.rs:3215` | render (paint_frame) |
+| 3 | style/syntax overlay | `src/overlay.rs:385` | render (Viewport) |
+| 4 | diagnostics overlay | `src/diag.rs:563` + `paint_line_markers` `:635` | render (Viewport) |
+| 5 | search wash overlay | `src/search.rs:492` | render (Viewport) |
+| 6 | completion popup anchor | completion overlay (byte→row) | render (Viewport) |
+| 7 | caret grid row | `src/editor.rs:3044` | render (paint_frame) |
+| 8 | local selection | `paint_local_selection`, `src/editor.rs:3241` | render (paint_frame) |
+| 9 | mode-line scroll indicator | `format_scroll_indicator`, `src/editor.rs:3803` | render (paint_frame) |
+| 10 | peer cursor + selection | `src/overlay_paint.rs:159` | **after** paint_frame |
+| 11 | click inverse | `activate_and_position`, `src/editor.rs:2233` | command/event |
+| 12 | auto-scroll clamp | `src/editor.rs:2866` | command/event |
+| 13 | paging / wheel / vertical motion | `src/editor_core.rs:1745/1779`, `src/editor.rs:2264`, `move_up/down` | command/event |
+
+Overlays (1,3,4,5,6) already thread through `Viewport`; sites 2,7,8,9 run in
+`paint_frame` directly; site 10 runs after `paint_frame` and re-derives
+`gutter_w`/`view_top` itself (`src/overlay_paint.rs:143-160`); sites 11–13 run
+during input dispatch, entirely outside any render frame.
+
+### 2.3 The renderer can reach the fold store
+
+`fold_registry: SharedFoldRegistry` is a field on `EditorCore`
+(`src/editor_core.rs:223`) and `EditorState` (`src/editor.rs:110`). Both the
+render path (`paint_frame`) and command-time code (`EditorCore` methods) reach it.
+The read surface (`src/fold.rs`): `FoldRegistry::folds(buf) -> Vec<ByteRange>`
+(`:327`, whole-buffer, sorted); `FoldStore::containing(p)` (`:171`, byte-space,
+`(start, end]`). **No line-space query exists** — Stage 2 adds it (§3). A fold's
+`start` = end-of-head-line content byte, `end` = end-of-last-hidden-line content
+byte, so `head_line = line_at_offset(start)`, hidden = `head_line+1 ..=
+line_at_offset(end)`. Use the fold's `(start,end]` convention, never the
+`ByteRange` struct doc's `[start,end)`.
+
+### 2.4 The gutter
+
+- `LineNumberMode` (`pmacs-protocol/src/message.rs:1173`; default **`Off`**) is
+  per-window (`Window.line_numbers`, `src/window.rs:188`). Number rule
+  `number_for(line, cursor_line)` (`:1197`) uses **raw-line `abs_diff`** for
+  relative distance.
+- `paint_line_number_gutter` (`src/editor.rs:3177`): `buffer_line = view_top + r`
+  (`:3215`) → `number_for(...)`; `cursor_line = line_at_offset(window.cursor)`
+  (`:3189`).
+- **Gutter width is 0 when line numbers are Off** (`Window::gutter_width`,
+  `src/window.rs:216`). When on, width = `decimal_digits(line_count)+2`, fixed to
+  the absolute count (no jitter; folding does not change `line_count`).
+- Diagnostic signs: `DiagnosticView::render` (`src/diag.rs:496`) →
+  `paint_line_markers` (`:635`): `gutter_w>0` draws the sign glyph in the gutter's
+  leading cell; **`gutter_w==0` falls back to a col-0 background** on the first
+  content cell (the "fake gutter"). So a dedicated sign cell only exists with a
+  gutter on (F3).
+
+### 2.5 The interactive-edit unfold seam (F5, correcting rev 1)
+
+Stage 1's `unfold_before_point_edit` (`src/editor_core.rs:1847`, reads
+`active_window().cursor`) is called at the top of the six `EditorCore` primitives;
+the shared `apply_active_edit` (`:1266`) they call does not itself unfold.
+
+- **Yank / query-replace** are **`apply_active_edit` callers** (local; never the
+  remote path): yank → `clipboard_paste`→`insert_bytes_over_region`→`apply_active_edit`
+  (`:2544/2570`); query-replace → `query_replace_apply_current`→`apply_active_edit`
+  (`:1129/1137`). They skip the six primitives, so they do not unfold today.
+- **Comment-toggle / yank-pop** take the Lua mutator path. The common entry is
+  **`run_buffer_edit`** (`src/lua_bindings/mod.rs:1305`), which dispatches to
+  `run_managed_edit` (`:1347`) *or* `run_bypass_edit` (`:1318`) on the
+  `bypass_intercept` flag; both call `apply_edit_skip_intercepts` then
+  `notify_buffer_edit_to_windows`. Hooking only `run_managed_edit` would let an
+  interactive `bypass_intercept` edit escape — hook `run_buffer_edit`.
+- **`InteractiveCommandOrigin`** (`src/editor.rs:53`) is an ephemeral, Lua-app-data
+  authenticated origin: `.current() -> Option<FrontendId>` is the frontend while an
+  interactive command runs; `.enter(fid)` returns a guard that restores on drop and
+  **clears even when a Lua command errors**. This is the scoped authority for the
+  Lua-path widening — no command-history inference needed.
+- **Undo/redo** reach the buffer through `notify_buffer_edit_to_windows` directly
+  (`add_history_methods`, `src/lua_bindings/mod.rs`), not `apply_active_edit` nor
+  `run_buffer_edit`.
+
+### 2.6 Scroll/viewport commands count raw lines; no recenter
+
+`move_page_down/up` (`src/editor_core.rs:1745/1779`), `scroll_window`
+(`src/editor.rs:2264`), `move_to_line` (`:708`), and the mode-line indicator all
+work in **raw source-line** space. There is no `beginning/end-of-buffer` command
+and **no recenter** (deferred, blocked on viewport facts; handoff §6). Stage 2
+inherits the recenter deferral.
+
+### 2.7 Windows and cursors are per-frontend; a frame renders several (F1, F2)
+
+Each attached frontend has its own `FrontendView`
+(`EditorCore.views: HashMap<FrontendId, FrontendView>`, `src/editor_core.rs:240`),
+registered at attach (`register_frontend_view`, `:540`) and cleared at detach
+(`unregister_frontend_view`, `:549`); windows/cursors are per-frontend instances a
+`FrontendView` references. Shared `EditorCore` motion (`move_up/down/page_*`) acts
+on `active_view()`/`active_window()` — **the acting frontend's own cursor**. The
+daemon holds the projection kind per frontend: exactly one of a grid `RenderState`
+or a semantic `SemanticRenderState` (`src/daemon.rs:875-881`), and **a grid and a
+semantic frontend can attach to the same buffer at once** — the fact that makes
+shared visible-line motion unsafe for a still-unfolded GPU session (F1).
+
+`paint_frame` (`src/editor.rs:2796`) renders **every** window in the acting
+frontend's layout; distinct windows carry distinct `window.buffer_id` /
+`view_top` / `text_view` (`:2922`, `reg.get(window.buffer_id)`), so a split can
+show two different buffers with only one folded. The after-frame presence pass
+likewise iterates recipient windows by `buffer_id` (`src/overlay_paint.rs:124`).
+Any per-frame-singleton fold map therefore leaks; the map must be **per rendered
+window** (F2). Command/event targeting matters too: wheel scrolling receives an
+explicit `win_id` and does not activate that pane, so its map belongs to the
+target window, not necessarily the active one (round-3 F1). Terminal windows
+never fold (Q#FD9) and are skipped.
+
+## 3. The shared visible-line map primitive (Q#FD12)
+
+Stage 2's spine is **one derivation/query primitive** — a `VisibleLineMap` type
+plus a builder — computed from `state.fold_registry.folds(buffer_id)` and the
+buffer's line offsets. It is **not one instance pinned to a frame**: it is built as
+**short-lived instances** wherever a source↔display mapping is needed (F4), and its
+home is a module usable from **both** the render path and `EditorCore` (not
+render-only). Candidate home: `src/fold_view.rs`, or a `FoldStore`
+convenience that returns hidden-line intervals given the line-offset table (the
+byte→line conversion then lives beside the `(start,end]` convention it must match).
+The byte-range store in `src/fold.rs` stays the single source of truth; the map is
+derived, never stored.
+
+### 3.1 Queries
+
+Folds may nest, share heads, or cross through arbitrary data-API ranges. The
+derivation unions overlapping or adjacent hidden intervals into sorted,
+non-overlapping **hidden components**. Each component
+`C = [first_hidden, last_hidden]` retains:
+
+- `head_line = first_hidden - 1`, the one visible line immediately before `C`;
+- `head_position`, the exact end-of-content byte of `head_line` (the
+  `range.start` that begins the earliest participating hidden interval).
+
+Adjacent intervals merge because the later fold's head is hidden by the earlier
+interval; keeping them separate would preserve a head that cannot render. The
+union of the components is the hidden-line set `H` (a line is hidden regardless
+of which fold owns it). The map answers:
+
+- `is_hidden(line) -> bool` — `line ∈ H`.
+- **`visible_head_of(line) -> line`** — for a hidden `line`, its component's
+  `head_line`; for a visible `line`, itself. This is the recursively resolved
+  **outermost visible head**, including when a crossing fold's own head is hidden
+  by an earlier fold. **Row-only** clamps use this: diagnostic signs, the
+  relative-number cursor anchor, and the `view_top` backward clamp. (Rev 1's
+  innermost `head_of` is removed — round-1 F1.)
+- **`visible_position_of(pos) -> Position` (round-2 F3, round-3 F2)** — for a byte
+  `pos` on a hidden line, its component's exact `head_position`; for a visible
+  `pos`, itself. This is equivalent to projecting to a containing fold's
+  `range.start` and repeating while that head is hidden, so it cannot return
+  another hidden position under crossing overlap. **Position** clamps — which
+  carry a column, not just a row — use this: the local caret, peer cursors, and
+  selection **endpoints**. A hidden point therefore lands at
+  `visible_head_of(line(pos))`'s end-of-content column, never at an arbitrary
+  column on the head.
+- `next_visible(line)` / `prev_visible(line)` — the next/previous visible line,
+  skipping whole folds (for the render walk and vertical motion).
+- `visible_between(a, b) -> isize` — signed count of visible lines from `a` to `b`
+  (relative/hybrid distance; paging).
+- **`clamp_view_top(line) -> line`** — if `line ∈ H`, `visible_head_of(line)`
+  (**backward**, so a fold at the top shows its head — acceptance 8); else `line`.
+  This replaces rev 1's forward `first_visible_at_or_after`, which skipped past the
+  fold and hid the head.
+
+Build cost is O(folds) with a byte→line lookup per fold — folds are O(top-level
+blocks) (parent B2), each needs a binary search into the render path's existing
+line-offset table, so O(folds · log lines); a linear merge with the table would be
+O(folds + lines) but is unnecessary given how few folds there are (**Bet B4**).
+
+### 3.2 Instances per phase and per window (F2, F4)
+
+The map is a **derivation/query primitive**, instantiated as short-lived instances
+— **never one singleton per frame** (F2, F4). Each instance is keyed on a specific
+window's `buffer_id`, its `TextView` line-offsets, and (for the render walk) its
+`view_top`:
+
+- **Render frame — one instance per rendered nonterminal window (F2).**
+  `paint_frame` (`src/editor.rs:2796`) iterates the acting frontend's windows; for
+  each **document** window it builds that window's map from
+  `state.fold_registry.folds(window.buffer_id)` and the window's line-offsets, then
+  (a) threads it to the overlay painters (1,3,4,5,6) as `Option<&'a
+  VisibleLineMap>` on a **lifetime-bearing `Viewport<'a>`** — a shared ref is
+  `Copy`, so `Viewport` stays `Copy` (F4); (b) passes the **same** per-window
+  instance to the in-`paint_frame` sites (2,7,8,9). The `View::render` signature
+  becomes `Viewport<'_>`; every construction site gains the field (non-fold callers
+  pass `None`). Terminal windows are skipped (Q#FD9). A split of two different
+  buffers gets two independent maps — no leakage. This is a settled compile-time
+  change across the `View` impls and `Viewport` constructions.
+- **After the frame — per recipient window (F2).** The peer-presence pass
+  (`src/overlay_paint.rs`, site 10) iterates recipient windows by `buffer_id`; for
+  each it derives/receives **that recipient window's** map and clamps peer cursors
+  (`visible_position_of`) / projects peer selection endpoints through it.
+- **Command / event — per target window, gated on the acting frontend (round-3
+  F1).** Click inverse, auto-scroll, paging, wheel, and vertical motion (sites
+  11–13, in `EditorCore` / `EditorState`) build a short-lived instance from
+  `state.fold_registry` + the **operation's target window** at call time — **only
+  when the acting frontend's `fold_projection_active` is set** (Q#FD21).
+  Motion, paging, and auto-scroll target the active window; click inverse and
+  wheel use their explicit `win_id` (wheel does not activate the pane under the
+  pointer). Otherwise they keep raw source-line behavior. "The map is already
+  built" (rev 1) was false — these run outside any frame.
+
+`view_top` **stays a source-line index** (Q#FD12, **Bet B5**), only ever set via
+`clamp_view_top` so it never rests on a hidden line — preserving the saveplace
+(`saveplace.lua:60`) and `_view_top`/`set_view_top` contracts. Visible-line
+*ordinals* are derived where needed, never stored.
+
+## 4. Collapse rendering (Q#FD13)
+
+`TextView::render` (`src/text_view.rs:207`) advances over **visible** lines via
+`next_visible`: row `r` shows the `r`-th visible source line at/after
+(already-visible) `view_top`; hidden lines are skipped; trailing rows clear as
+today. A visible head line renders its real content then a trailing ellipsis
+marker (` …`) in the **content area** (clipped like any long line) — the
+authoritative, layout-neutral fold indicator. Overlays (1,3,4,5,6) read the same
+map through `Viewport`, so a span/wash/sign/anchor on a hidden line is simply not
+painted (its row does not exist) and one on a visible line lands on the right row.
+Folding is **not** an overlay — overlays cannot delete rows.
+
+## 5. Gutter: line numbers + fold glyph (Q#FD14, Q#FD20)
+
+**Line numbers (Q#FD14).** `paint_line_number_gutter` (`src/editor.rs:3177`) walks
+visible lines (row `r` → the `r`-th visible line). **Absolute** shows that line's
+raw `line+1` (hidden numbers just do not appear — the column jumps from the head's
+number to the first post-fold number). **Relative/Hybrid** measure distance in
+**visible** lines: `visible_between(anchor, row_line)`, where the cursor **anchor**
+is `visible_head_of(cursor_line)` (F1 — the cursor may be on a hidden shared-fold
+line). Absolute still needs the raw line, so the walk carries both the raw source
+line and the visible ordinal. Gutter width is unchanged (§2.4).
+
+**Fold glyph (Q#FD20, F3).** Conditioned on a gutter existing:
+- **Line numbers off (`gutter_w==0`, default):** no sign cell exists, so the fold
+  marker is the **content-area ellipsis only**; the diagnostic col-0 background
+  fallback is unchanged.
+- **Line numbers on (`gutter_w>0`):** on a head-line row, draw a fold glyph in the
+  gutter's col-0 sign cell **unless a diagnostic clamps there** (Q#FD15) —
+  diagnostic wins (an error inside the fold is higher-signal).
+
+This keeps the gutter width rule intact and adds no column. Making the gutter
+marker **unconditional** would require a **dedicated fold sign column** and an
+acknowledged layout change; deferred (§9). The parent §7's promise is thus honored
+*when a gutter is present*, with the ellipsis as the universal indicator.
+
+## 6. Diagnostic signs on hidden lines (Q#FD15)
+
+`DiagnosticView::render` (`src/diag.rs:496`): before recording a marker for source
+`line` (`:563/:570`), if `is_hidden(line)` **remap to `visible_head_of(line)`**
+(clamp to the outermost visible head, F1), not drop. The existing
+most-severe-per-row merge (`:570`) then makes the head row show the most severe
+sign among the head and every hidden line under its fold. Clamp (not drop)
+preserves the "there is a problem inside this collapsed region" signal.
+`DiagnosticView` reaches the map through `Viewport` (§3.2), needing no separate
+handle.
+
+## 7. Caret, selection, peer presence, viewport, and motion (Q#FD16, Q#FD17, Q#FD18, Q#FD21)
+
+**Frontend scoping (Q#FD21, F1).** Render-time clamps (this section's caret,
+selection, peer, gutter) live in the grid `paint_frame`/presence path, which a
+semantic/GPU session never enters, so they are **already grid-only**. The
+**command/event-time** reckoning below (motion, paging, wheel, click, auto-scroll)
+runs in **shared** `EditorCore`/`EditorState` code, so each such site is **gated on
+the acting frontend's `fold_projection_active`** (a `FrontendView` flag,
+`src/editor_core.rs:240`, set at attach from
+`session_state.negotiated_capabilities.semantic_render`, the same selected-render
+bit that allocates `RenderState` vs `SemanticRenderState`: grid ⇒ true in Stage 2;
+semantic/GPU ⇒ false until Stage 3; LOCAL ⇒ true; cleared with the view at detach).
+When the flag is false the site keeps its current raw-line behavior — so a GPU
+session on the same shared-fold buffer never skips a source line it still displays.
+
+**Caret clamp (Q#FD16, F3).** Caret grid position (`src/editor.rs:3033-3044`): if
+the logical cursor's line `is_hidden`, render at **`visible_position_of(cursor)`**
+— the hidden component's `head_position`, i.e. the visible head row at the head's
+end-of-content column (not an arbitrary or still-hidden column, F3). Satisfies
+the parent's per-cursor render-time invariant (Q#FD3), including the shared-store
+case.
+
+**Local selection (Q#FD16, F2/F3).** `paint_local_selection` (`src/editor.rs:3241`):
+each selection **endpoint** on a hidden line projects via `visible_position_of`;
+hidden interior cells are dropped; the visible portion paints on the visible head
+row and the visible tail rows, contiguous on screen.
+
+**Peer presence (Q#FD16, F2/F3).** `src/overlay_paint.rs:159`: a peer cursor on a
+hidden line clamps via `visible_position_of`; peer selection endpoints project the
+same way and hidden interiors drop. The map is the **recipient window's** map
+(§3.2), built for that window's buffer.
+
+**Click inverse (Q#FD16, Q#FD21).** `activate_and_position` (`src/editor.rs:2233`):
+on a fold-projecting frontend, build the map from the explicit target `win_id`;
+grid row `k` maps to that window's `k`-th visible line, so a click never lands on
+a hidden line.
+
+**Viewport / scroll (Q#FD18, Q#FD21).** On a fold-projecting frontend: `view_top`
+is set only via `clamp_view_top` (backward to the head for a hidden candidate, F1);
+the active window's paging (`move_page_down/up`) and auto-scroll-to-cursor clamp
+(`src/editor.rs:2866`), plus wheel `scroll_window(win_id)` on its explicit target
+window, advance and bound by **visible** lines
+(`next_visible`/`prev_visible`, `visible_between`); `move_to_line`/goto-line targets
+a raw line and, if hidden, clamps `view_top` so the target's **head** is visible (no
+auto-unfold — deferred search-reveal, §9); the **mode-line scroll indicator**
+(`format_scroll_indicator`, `src/editor.rs:3803`) computes All/Top/Bot/% in
+**visible-line** space (visible total, `view_top`'s visible ordinal, cursor's
+visible ordinal). Recenter stays deferred (§2.6).
+
+**Vertical line-motion (Q#FD17 — RULED: include; Q#FD21-scoped).** On a
+fold-projecting frontend, `next-line`/`prev-line` (and arrows) step to the adjacent
+**visible** line (`next_visible`/`prev_visible`), so a collapsed region is one
+motion step and the cursor never rests hidden. **If motion begins from a hidden
+logical cursor** (a shared fold or a goto-line into a fold), it **first normalizes
+to `visible_position_of(cursor)`**, then steps. The render-time caret clamp (Q#FD16)
+backstops the pre-motion shared-fold frame. On a non-fold-projecting frontend these
+retain raw-line motion. All reuse the command-time per-window map (§3.2).
+
+## 8. Interactive-Lua-command unfold widening (Q#FD19)
+
+Widen the pre-edit unfold beyond the six `dispatch_key` primitives to the
+interactive Lua commands, **local-interactive only** (never the remote/optimistic
+CRDT path — parent Stage 3).
+
+- **`apply_active_edit` funnel (yank, query-replace, and the six).** Move the
+  pre-edit unfold to the **top of `apply_active_edit`** (`src/editor_core.rs:1266`),
+  keyed on the active frontend's point. This subsumes the six primitives'
+  individual calls (retired — one funnel) and covers yank + query-replace for free.
+  `apply_active_edit` is never the remote-apply path, so this funnel is inherently
+  local.
+- **Interactive Lua-mutator funnel (comment-toggle, yank-pop).** Hook the **common
+  `run_buffer_edit`** (`src/lua_bindings/mod.rs:1305`) — above both
+  `run_managed_edit` and `run_bypass_edit`, so an interactive `bypass_intercept`
+  edit does not escape (F5). Unfold **only when all** hold: (i)
+  `InteractiveCommandOrigin.current()` is `Some(f)` (`src/editor.rs:53`); (ii) the
+  edited buffer **is `f`'s active-window buffer** (an explicit inactive-buffer Lua
+  mutation stays programmatic — no unfold); (iii) `edit.range.start` is inside a
+  fold. Condition (i)+(ii) is what distinguishes an interactive command's edit at
+  the point from a plugin's/data-API's programmatic edit (matching Stage 1's
+  data-API exemption).
+- **Excluded (Stage 3):** the remote/optimistic-CRDT apply path
+  (`notify_buffer_edit`, `src/editor_core.rs:1364`). A remote peer's edit inside my
+  fold must not unfold it; a GPU user's own optimistic edit is the parent's Stage 3
+  obligation.
+- **Undo/redo — DEFERRED (F5 ruling).** `undo`/`redo` reach
+  `notify_buffer_edit_to_windows` directly; their unfold behavior is **explicitly
+  deferred** (§9), not decided at implementation time.
+
+Each widened behavior is **bite-verified** (a test failing without the widening).
+
+## 9. Deferred (named)
+
+- **Stage 3 (GPU):** GPU collapse, caret/hit-test fold-awareness at TUI parity, the
+  `BufferSnapshot` fold-mirror clear (parent R2-4), and CRDT-origin / GPU-optimistic
+  interactive unfold (parent R2-3).
+- **Undo/redo unfold** (F5 ruling) — deferred.
+- **Recenter** and any frontend scroll control — blocked on viewport facts; Arc 8
+  adjacent (§2.6).
+- **Search-reveal** — a match inside a fold auto-unfolds; parent §11 "Stage 2+";
+  goto-line likewise does not auto-unfold (§7).
+- **A dedicated gutter fold column** (unconditional marker) — its width recompute /
+  layout change is deferred; Q#FD20 uses the conditional sign cell.
+- **Fold-aware horizontal/word motion** and screen-line editing beyond vertical
+  line-motion.
+- **Persisted folds, `hide-level N`, auto-fold-on-open**; fold-store revalidation on
+  revert/reload (parent §11; v1 still drops it).
+
+## 10. Bets
+
+- **B4** The per-instance visible-line map is cheap: O(folds) build, one byte→line
+  binary search per fold into the render path's existing line-offset table
+  (O(folds · log lines)); folds are few, so per-window and command-time
+  re-derivation is negligible. FALSIFIABLE on a pathological many-fold buffer
+  (mitigation: cost scales with folds, not lines).
+- **B5** `view_top` stays a source-line index (clamped via `clamp_view_top`) —
+  less churn than a visible-ordinal coordinate space, and preserves saveplace /
+  `set_view_top`.
+- **B6** No wire/protocol change: `FoldState` (Stage 1) untouched; the TUI collapse
+  is daemon-side; a semantic GPU session still gets `FoldState` and renders nothing
+  new until Stage 3.
+- **B7** Threading `Option<&'a VisibleLineMap>` on a lifetime-bearing `Viewport<'a>`
+  keeps `Viewport: Copy`. FALSIFIABLE if a `View` impl or construction site cannot
+  satisfy the lifetime; fallback is to settle `Viewport` as non-`Copy` explicitly.
+- **B8** A `fold_projection_active` flag on `FrontendView`, set from
+  `session_state.negotiated_capabilities.semantic_render` in the same attach
+  transaction that selects the grid vs semantic render state (and initialized
+  true for LOCAL), cleanly scopes command-time visible-line reckoning to grid
+  frontends. Every non-daemon `FrontendView` construction must choose the
+  projection explicitly; it is never inferred from `FrontendId`.
+
+## 11. Acceptance — Stage 2
+
+Tests assert on the **rendered cell grid** through a real-daemon / real-TUI grid
+harness (the vterm-Stage-2 real-PTY smoke and the UX-gutter daemon-acceptance are
+the precedents), sized for macOS startup + long temp-path width (handoff §5).
+
+1. **Collapse.** Fold a region; the frame omits the hidden lines, the head shows
+   text + ellipsis, rows below shift up, content row count = visible-line count.
+2. **Head marker, both gutter states (F3).** With line numbers **off**: the head
+   shows the ellipsis and **no gutter glyph** (width unchanged, 0). With a
+   line-number mode **on**: the head shows the ellipsis **and** the gutter fold
+   glyph (unless a diagnostic clamps there — then the diagnostic sign).
+3. **Line numbers.** Absolute skips hidden numbers (head's number, then first
+   post-fold number, no gap-fill). Relative/Hybrid distance is measured in
+   **visible** lines across a fold, anchored on the visible cursor head.
+4. **Diagnostic clamp (F1).** A diagnostic on a hidden line surfaces on the
+   **outermost visible head** row; most-severe wins across the head and all its
+   hidden lines (including a diagnostic on a *nested* inner-fold line clamping to
+   the outer head).
+5. **Nested-fold / shared cursor (F1).** With a fold nested inside another and the
+   logical cursor on a deeply-hidden line (folded via the shared store from a
+   second frontend), the caret renders on the **outermost** visible head; relative
+   numbers anchor there; a `view_top` set into the nest clamps **backward** to that
+   head.
+6. **Caret/selection/peer column projection (F2/F3).** A caret whose hidden line's
+   column **differs materially from the head's end-of-content** renders at the head
+   row **and the head's end-of-content column** (`visible_position_of` →
+   `range.start`), never at the raw hidden column. A local selection spanning a fold
+   projects its endpoints and paints on the visible head + visible tail rows,
+   nothing on hidden rows. A peer cursor on a hidden line clamps the same way; peer
+   selection endpoints project, interiors drop. A click never selects a hidden line.
+   Repeat the caret/peer/endpoint assertions with **crossing folds** (A hides
+   lines 1–3; B's head is hidden on line 2 and B hides through line 5): a point on
+   line 5 resolves to A's visible head position, never B's hidden
+   `range.start`.
+7. **Ordinary overlays across a fold (F2).** A style/syntax span and a search wash
+   on lines straddling a fold paint only on the visible rows, correctly aligned;
+   the completion popup anchored below a fold lands on the right visible row.
+8. **Viewport / paging / indicator (F2).** Page-down advances by a screenful of
+   **visible** lines across a fold; `view_top` never rests hidden; a fold at the top
+   clamps to its head; goto-line to a hidden line leaves its head visible; the
+   mode-line indicator reports Top/Bot/% in visible-line space.
+9. **Vertical motion (Q#FD17).** `next-line`/`prev-line` step across a collapsed
+   region as one motion; starting from a hidden logical cursor, motion first
+   normalizes to the visible head; the cursor never rests hidden.
+10. **Interactive unfold widening (Q#FD19).** A **yank**, a **query-replace**
+    replacement, and a **comment-toggle** whose edit point is inside a fold each
+    unfold it before the edit is visible; a **bypass-intercept** interactive edit at
+    the point also unfolds (proving the `run_buffer_edit` seam). A **programmatic**
+    `buf:insert` — and an interactive command mutating an **inactive** buffer — do
+    **not** unfold. **Undo/redo do not unfold** (deferred). Each assertion is
+    **bite-verified**; the test documents CRDT-origin unfold as the Stage 3
+    obligation.
+11. **No wire/protocol change.** A semantic session still receives `FoldState`
+    exactly as in Stage 1; the protocol version is unchanged; the GPU render path is
+    untouched (the Stage-1 `FoldState` producer transitions test passes verbatim).
+12. **Shared store, independent viewports.** Two TUI windows on one buffer both
+    render the same fold collapsed, each with its own correct `view_top`, gutter, and
+    caret.
+13. **Frontend-scoped motion (F1, Q#FD21).** A **grid TUI** and a **semantic**
+    session attach to one buffer with a fold in the store. Down/Up and page-down on
+    the TUI move its cursor by **visible** lines (skipping the fold); the same
+    commands issued by the semantic session move its cursor by **raw** source lines
+    (it still displays them). Attach then detach the semantic session and re-check
+    the TUI is unaffected — the flag is per-`FrontendView`.
+14. **Split of different buffers, one folded (F2).** A vertical split shows buffer A
+    (with a fold) and buffer B (no fold). A's window collapses; **B's window is
+    byte-for-byte identical to the no-fold baseline** (no active-buffer/map leakage);
+    peer presence painted into B's window uses B's map. Keep folded A active and
+    wheel-scroll over inactive B: focus stays on A, while B's `view_top` and
+    cursor advance through **B's** raw-visible lines using B's map, never A's.
+
+## 12. Gates (Stage 2)
+
+`cargo fmt --check`; `cargo clippy --workspace --all-targets -- -D warnings` (own
+step); `cargo test --lib`; `cargo test --lib --features crdt`;
+`tests/folding_acceptance.rs` (Stage-1 suite, stays green) + new
+`tests/folding_stage2_acceptance.rs` (default + CRDT); `cargo test --test
+m4_acceptance -- --skip basedpyright`; `PMACS_REQUIRE_GPU=1 cargo test -p pmacs-gpu`
+(stays green — Stage 2 does not touch the GPU); the workspace sweep as one
+invocation; `git diff --check`. New behavioral acceptance is bite-verified with
+`scripts/bite`; timing-flaky tests are rerun isolated before treating a sweep
+failure as a regression (handoff §3).
+
+## 13. Numbered decisions (continuing the parent's Q#FD scheme)
+
+- **Q#FD12** One shared visible-line map **primitive** (derivation + queries from
+  `folds()` + line offsets), instantiated as short-lived instances **per rendered
+  window and per phase** (render via `Option<&'a VisibleLineMap>` on `Viewport<'a>`
+  preserving `Copy`; after-frame per recipient window; command-time per operation
+  target window), never a per-frame singleton (F2), home usable from `EditorCore`.
+  `view_top` stays a source-line index, set only via `clamp_view_top`. Every
+  consumer in the §2.2 census routes through it. (§3)
+- **Q#FD13** Collapse in `TextView::render` (row `r` → `r`-th visible line); head
+  renders content + trailing ellipsis; folding is not an overlay. (§4)
+- **Q#FD14** Line numbers walk visible lines; Absolute uses raw `line+1`,
+  Relative/Hybrid measure **visible**-line distance anchored on `visible_head_of`
+  of the cursor; gutter width unchanged. (§5)
+- **Q#FD15** A diagnostic on a hidden line clamps to `visible_head_of` (outermost
+  visible head), most-severe merge; not dropped. (§6)
+- **Q#FD16** Render-time **position** clamp via `visible_position_of` (the merged
+  hidden component's visible `head_position` — head row **and** head
+  end-of-content column, including crossing overlaps) for the caret, peer cursors,
+  and selection endpoints; hidden selection interiors drop;
+  diagnostics and the relative-number anchor use the row-only `visible_head_of`;
+  click inverse maps grid rows to visible lines. (§7)
+- **Q#FD17** *(ruled: include)* Vertical line-motion steps by visible lines; motion
+  from a hidden cursor first normalizes to the visible head. Scoped by Q#FD21. (§7)
+- **Q#FD18** Viewport/paging/auto-scroll/indicator reckon in visible lines;
+  `view_top` set via `clamp_view_top` (backward to the head); goto-line leaves a
+  hidden target's head visible; recenter deferred. Scoped by Q#FD21. (§7)
+- **Q#FD19** Interactive-unfold widening hooks **local** funnels only:
+  `apply_active_edit` (top; subsumes the six; covers yank + query-replace) and the
+  common `run_buffer_edit` gated on `InteractiveCommandOrigin.current()` **and**
+  the edit targeting the invoking frontend's active-window buffer **and**
+  `edit.range.start` inside a fold; the remote/optimistic-CRDT `notify_buffer_edit`
+  path is excluded (Stage 3); **undo/redo unfold is deferred**. (§8)
+- **Q#FD20** Fold gutter glyph is **conditional on a gutter existing**: off ⇒
+  ellipsis only; on ⇒ col-0 sign cell on the head row with diagnostic priority. A
+  dedicated fold column (unconditional marker + layout change) is deferred. (§5)
+- **Q#FD21** Fold projection is **per-frontend**: a `fold_projection_active` flag on
+  `FrontendView`, set at attach from the negotiated selected-render bit (grid ⇒
+  true in Stage 2; semantic/GPU ⇒ false until Stage 3; LOCAL ⇒ true), cleared at
+  detach. All
+  command/event-time visible-line reckoning (motion, paging, wheel, click,
+  auto-scroll) is gated on the acting frontend's flag, while the map derives from
+  the operation's target window (active for motion/paging/auto-scroll; explicit
+  `win_id` for click/wheel); render-time clamps are already grid-path-only. Keeps
+  a simultaneous unfolded GPU session's cursor from skipping lines it still
+  displays without leaking another pane's fold map. (§7, §2.7)
+
+## 14. Branch and PR plan
+
+Branch **`folding-tui`**, worktree `../pmacs-folding-tui`, off canonical `main` @
+`c49a8c7` (folding Stage 1 / #142 merged). This framing is the opening commits (rev
+1 → rev 4); Stage 2 implements on this same branch and opens as the second folding
+PR. One feature, one branch, one PR — Stage 3 (GPU) is a separate branch/PR off the
+resulting main.
+
+Housekeeping from #142 (retire the `docs/active-work.md` folding lane + refresh
+`docs/agent-handoff.md` §1) is **ready as the separate, unmerged docs PR #147**,
+kept out of `folding-tui` (ruled).

docs/gpu-initial-target-framing.md

@@ -628,13 +628,14 @@ and the complete #141 lifecycle fixture coverage together.
 Post-second-review verification on 2026-07-24:
 
 - `cargo fmt --check` and strict workspace Clippy passed.
-- Library gates passed 1,801 default and 1,978 CRDT tests.
+- Library gates passed 1,815 default and 1,992 CRDT tests.
 - The named initial-target gate passed 1 default and 14 CRDT tests; the
   underlying GPU invocation suite passed 14 CRDT tests.
+- Folding Stage 2 passed 48 CRDT tests after integrating current `main`.
 - M4 passed 121 tests with the documented basedpyright skip; required real-GPU
   tests passed 152.
 - Vterm Stage 3 passed 5 default and 7 CRDT tests.
-- The isolated-config workspace CRDT sweep passed 3,272 tests across 87 suites,
+- The isolated-config workspace CRDT sweep passed 3,334 tests across 88 suites,
   with 29 ignored and the documented basedpyright case filtered.
 - A coherent release build launched two concurrent real Wayland/Vulkan GPU
   windows on one daemon, targeting distinct `alpha` and `beta` files. Both

src/completion.rs

@@ -684,7 +684,7 @@ struct PopupRect {
 /// the viewport or nothing fits.
 fn resolve_popup_rect(
     buf: &Buffer,
-    viewport: Viewport,
+    viewport: Viewport<'_>,
     anchor: Position,
     rows: &[PopupCandidate],
 ) -> Option<PopupRect> {
@@ -700,10 +700,12 @@ fn resolve_popup_rect(
     let line_offsets = crate::diag::compute_line_offsets(&source);
     let start_line = crate::diag::line_at_offset(&line_offsets, viewport.buffer_start as u32);
     let anchor_line = crate::diag::line_at_offset(&line_offsets, anchor);
-    if anchor_line < start_line {
+    // Arc 6 Stage 2: the popup anchors on the anchor byte's VISIBLE row,
-        return None; // anchor scrolled above the viewport
+    // so a completion below a collapsed region lands on the right row;
-    }
+    // an anchor inside a collapse has no row and paints nothing.
-    let anchor_row = anchor_line - start_line;
+    let Some(anchor_row) = viewport.row_offset_of(start_line as usize, anchor_line as usize) else {
+        return None; // anchor scrolled above the viewport, or collapsed
+    };
     let max_rows = viewport.cell_size.rows;
     let max_cols = viewport.cell_size.cols;
     if anchor_row >= max_rows || max_cols == 0 {
@@ -820,7 +822,7 @@ impl View for CompletionView {
         "completion-popup"
     }
 
-    fn render(&mut self, buf: &Buffer, viewport: Viewport, cells: &mut CellGrid<'_>) {
+    fn render(&mut self, buf: &Buffer, viewport: Viewport<'_>, cells: &mut CellGrid<'_>) {
         // Snapshot under the lock, then drop it before touching the rope.
         let (anchor, rows_data, selected_in_window): (Position, Vec<PopupCandidate>, usize) = {
             let guard = self.popup.lock().expect("completion popup poisoned");

src/daemon.rs

@@ -1757,7 +1757,19 @@ fn handle_session_established(
     initial_target: Option<InitialTarget>,
     mut write_stream: UnixStream,
 ) {
-    let fresh_view = build_fresh_frontend_view(editor);
+    // Register the frontend's view (M10.8 Day 3: fresh scratch
+    // buffer view; future milestones may clone LOCAL's view or
+    // take an explicit initial-buffer argument).
+    //
+    // Arc 6 Stage 2 (Q#FD21): the projection is decided in this same
+    // attach transaction and from the same bit that selects a grid
+    // `RenderState` vs a `SemanticRenderState` below — a grid session
+    // collapses folds, a semantic one keeps raw-line reckoning until
+    // Stage 3.
+    let fresh_view = build_fresh_frontend_view(
+        editor,
+        !session_state.negotiated_capabilities.semantic_render,
+    );
     {
         let mut core = editor.core.borrow_mut();
         core.register_frontend_view(frontend_id, fresh_view);
@@ -2920,7 +2932,13 @@ fn align_semantic_window_to_buffer(
     }
 }
 
-fn build_fresh_frontend_view(editor: &mut EditorState) -> crate::window::FrontendView {
+fn build_fresh_frontend_view(
+    editor: &mut EditorState,
+    // Arc 6 Stage 2 (Q#FD21, Bet B8): whether this session's display
+    // collapses folds. Passed explicitly from the negotiated
+    // selected-render bit at the call site — never inferred here.
+    fold_projection: bool,
+) -> crate::window::FrontendView {
     use crate::text_view::TextView;
     use crate::window::{FrontendView, Layout, Window, WindowId};
     let mut core = editor.core.borrow_mut();
@@ -2949,6 +2967,7 @@ fn build_fresh_frontend_view(editor: &mut EditorState) -> crate::window::Fronten
     FrontendView {
         layout: Layout::single(id),
         active: id,
+        fold_projection,
     }
 }
 
@@ -3214,7 +3233,7 @@ mod tests {
         let semantic_peer = FrontendId(20);
         let live_grid_peer = FrontendId(21);
         let dead_grid_peer = FrontendId(22);
-        let semantic_view = build_fresh_frontend_view(&mut editor);
+        let semantic_view = build_fresh_frontend_view(&mut editor, false);
         editor
             .core
             .borrow_mut()
@@ -3865,6 +3884,7 @@ mod tests {
                 FrontendView {
                     layout: Layout::single(wid),
                     active: wid,
+                    fold_projection: true,
                 },
             );
         }
@@ -3995,7 +4015,9 @@ mod tests {
 
         let mut editor = EditorState::new();
         let fid = FrontendId(99);
-        let view = build_fresh_frontend_view(&mut editor);
+        // Both these fixtures model a SEMANTIC session (Q#FD21: no fold
+        // projection until Stage 3).
+        let view = build_fresh_frontend_view(&mut editor, false);
         editor.core.borrow_mut().register_frontend_view(fid, view);
 
         let before = editor
@@ -4056,7 +4078,9 @@ mod tests {
 
         // Attach: window shares LOCAL's active (scratch).
         let fid = FrontendId(99);
-        let view = build_fresh_frontend_view(&mut editor);
+        // Both these fixtures model a SEMANTIC session (Q#FD21: no fold
+        // projection until Stage 3).
+        let view = build_fresh_frontend_view(&mut editor, false);
         editor.core.borrow_mut().register_frontend_view(fid, view);
         assert_eq!(
             editor

src/desktop.rs

@@ -435,6 +435,8 @@ pub fn restore_into(
             FrontendView {
                 layout: Layout { root },
                 active,
+                // Desktop restore rebuilds LOCAL's grid view (Q#FD21).
+                fold_projection: true,
             },
         );
         active

src/diag.rs

@@ -493,7 +493,7 @@ impl View for DiagnosticView {
         "diagnostic"
     }
 
-    fn render(&mut self, buf: &Buffer, viewport: Viewport, cells: &mut CellGrid<'_>) {
+    fn render(&mut self, buf: &Buffer, viewport: Viewport<'_>, cells: &mut CellGrid<'_>) {
         // Snapshot the diagnostics under the lock and drop it
         // immediately so we don't hold the lock through rendering
         // (rendering touches the rope, which could in principle
@@ -557,20 +557,23 @@ impl View for DiagnosticView {
                 if line >= total_lines {
                     break;
                 }
-                if line < start_line_buf {
+                let row_offset = viewport.row_offset_of(start_line_buf as usize, line as usize);
+                let Some(sign_row) = sign_row_for(viewport, start_line_buf, line) else {
                     continue;
-                }
+                };
-                let row_offset = line - start_line_buf;
+                if sign_row >= max_rows {
-                if row_offset >= max_rows {
                     break;
                 }
                 // Record the line marker before any byte-range work:
                 // zero-width ranges (`byte_end <= byte_start` below)
                 // skip the underline but still mark the line.
                 line_markers
-                    .entry(row_offset)
+                    .entry(sign_row)
                     .and_modify(|s| *s = (*s).min(diag.severity))
                     .or_insert(diag.severity);
+                let Some(row_offset) = row_offset else {
+                    continue;
+                };
                 let line_start = line_offsets[line as usize];
                 let line_end = line_offsets
                     .get(line as usize + 1)
@@ -625,6 +628,25 @@ impl View for DiagnosticView {
     }
 }
 
+/// The grid row a diagnostic on source `line` marks (Arc 6 Stage 2,
+/// Q#FD15).
+///
+/// Its own row normally; when the line is collapsed away, the fold's
+/// **outermost visible head** row — a clamp, not a drop, so "there is a
+/// problem inside this collapsed region" survives the collapse (the
+/// most-severe-per-row merge then makes the head show the worst severity
+/// among itself and every line its fold hides). `None` when neither has
+/// a row in this viewport. Only the *sign* clamps: a squiggle needs a
+/// real row, so a hidden line contributes no underline.
+fn sign_row_for(viewport: Viewport<'_>, start_line: u32, line: u32) -> Option<u32> {
+    viewport
+        .row_offset_of(start_line as usize, line as usize)
+        .or_else(|| {
+            let map = viewport.folds?;
+            viewport.row_offset_of(start_line as usize, map.visible_head_of(line as usize))
+        })
+}
+
 /// Paint one severity marker per diagnostic line (UX gutter sub-arc 2).
 ///
 /// When the window reserves a gutter (`gutter_w > 0`), draw the severity
@@ -1046,6 +1068,7 @@ mod tests {
                 cell_origin: CellCoord::new(0, 0),
                 cell_size: CellSize::new(1, 10),
                 gutter_w: 0,
+                folds: None,
             },
             &mut grid,
         );
@@ -1110,6 +1133,7 @@ mod tests {
                 cell_origin: CellCoord::new(0, 0),
                 cell_size: CellSize::new(3, 10),
                 gutter_w: 0,
+                folds: None,
             },
             &mut grid,
         );
@@ -1185,6 +1209,7 @@ mod tests {
                 cell_origin: CellCoord::new(0, 2),
                 cell_size: CellSize::new(3, 8),
                 gutter_w: 2,
+                folds: None,
             },
             &mut grid,
         );
@@ -1253,6 +1278,7 @@ mod tests {
                 cell_origin: CellCoord::new(0, 0),
                 cell_size: CellSize::new(2, 10),
                 gutter_w: 0,
+                folds: None,
             },
             &mut grid,
         );

src/editor.rs

@@ -41,7 +41,7 @@ use crate::protocol::{
 use crate::terminal::TerminalSnapshot;
 use crate::terminal::view::TerminalViewKey;
 use crate::view::{View, Viewport};
-use crate::window::{Rect, WindowId};
+use crate::window::{LineNumberMode, Rect, WindowId};
 
 /// Ephemeral authenticated origin for one interactive command invocation.
 ///
@@ -2230,8 +2230,20 @@ impl EditorState {
         core.set_active_window_id(win_id);
         let view_top = core.windows[&win_id].view_top;
         let buffer_id = core.windows[&win_id].buffer_id;
-        let display_row = view_top.saturating_add(local_row as usize);
+        // Arc 6 Stage 2 (Q#FD16/FD21): grid row `k` in this window shows
-        let target = crate::view::DisplayCoord::new(display_row as u32, local_col);
+        // its `k`-th VISIBLE line, so the inverse must walk the same way
+        // — a click can then never land on a collapsed line. The map is
+        // the CLICKED window's (round-3 F1), not the previously active
+        // one's.
+        let folds = core.fold_map_for_window(win_id);
+        let display_row = match folds.as_ref() {
+            Some(map) => map.nth_visible_from(view_top, local_row as usize),
+            None => view_top.saturating_add(local_row as usize),
+        };
+        let Ok(display_row) = u32::try_from(display_row) else {
+            return;
+        };
+        let target = crate::view::DisplayCoord::new(display_row, local_col);
         let pos = {
             let registry = core.registry.clone();
             let reg = registry.borrow();
@@ -2263,23 +2275,33 @@ impl EditorState {
     /// `mouse-wheel-mode` and every modern editor's wheel behaviour.
     fn scroll_window(&mut self, win_id: WindowId, delta: i32) {
         let mut core = self.core.borrow_mut();
+        // Arc 6 Stage 2 (Q#FD18/FD21, round-3 F1): a wheel event names
+        // the pane under the pointer and does NOT activate it, so the map
+        // must come from `win_id` — deriving the active window's would
+        // project a folded buffer onto an unfolded neighbour. The
+        // projection *policy* still comes from the acting frontend.
+        let folds = core.fold_map_for_window(win_id);
         let line_count = core.windows[&win_id].text_view.line_count();
         let max_top = line_count.saturating_sub(1);
         let old_top = core.windows[&win_id].view_top;
         let scroll_up = delta < 0;
         let magnitude = delta.unsigned_abs() as usize;
-        let new_top = if scroll_up {
+        let new_top = match folds.as_ref() {
-            old_top.saturating_sub(magnitude)
+            Some(map) if scroll_up => map.nth_visible_back(old_top, magnitude),
-        } else {
+            Some(map) => map
-            old_top.saturating_add(magnitude).min(max_top)
+                .nth_visible_from(old_top, magnitude)
+                .min(map.visible_head_of(max_top)),
+            None if scroll_up => old_top.saturating_sub(magnitude),
+            None => old_top.saturating_add(magnitude).min(max_top),
         };
         // Effective view delta — buffer-boundary clamping may shrink
         // the requested move, so the cursor only follows by however
-        // many lines the view actually shifted.
+        // many lines the view actually shifted (counted in VISIBLE
-        let view_shift = if scroll_up {
+        // lines once this window folds).
-            old_top.saturating_sub(new_top)
+        let view_shift = match folds.as_ref() {
-        } else {
+            Some(map) => map.visible_distance(old_top, new_top),
-            new_top.saturating_sub(old_top)
+            None if scroll_up => old_top.saturating_sub(new_top),
+            None => new_top.saturating_sub(old_top),
         };
         let buffer_id = core.windows[&win_id].buffer_id;
         let new_cursor = {
@@ -2289,10 +2311,13 @@ impl EditorState {
                 let aw = &core.windows[&win_id];
                 let cur = aw.text_view.pos_to_display(buf, aw.cursor)?;
                 let cur_row = cur.row as usize;
-                let target_row_usize = if scroll_up {
+                let target_row_usize = match folds.as_ref() {
-                    cur_row.saturating_sub(view_shift)
+                    Some(map) if scroll_up => map.nth_visible_back(cur_row, view_shift),
-                } else {
+                    Some(map) => map
-                    cur_row.saturating_add(view_shift).min(max_top)
+                        .nth_visible_from(cur_row, view_shift)
+                        .min(map.visible_head_of(max_top)),
+                    None if scroll_up => cur_row.saturating_sub(view_shift),
+                    None => cur_row.saturating_add(view_shift).min(max_top),
                 };
                 let target_row = u32::try_from(target_row_usize).ok()?;
                 aw.text_view
@@ -2316,6 +2341,12 @@ impl EditorState {
 /// readline / Emacs default and is what most terminal users expect.
 const SCROLL_LINES: i32 = 3;
 
+/// Gutter marker drawn on a collapsed region's head row (Arc 6 Stage 2,
+/// Q#FD20). Occupies the gutter's leading pad cell — the same cell the
+/// diagnostic sign uses — so it adds no column and changes no width; it
+/// therefore only appears when a line-number mode reserves a gutter.
+const FOLD_GUTTER_GLYPH: char = '▸';
+
 /// Shared outer/content geometry consumed by terminal paint and PTY resize.
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub(crate) struct WindowPlacement {
@@ -2860,6 +2891,13 @@ pub fn paint_frame(
             && let Some(buf_id) = buf_id
             && let Ok(buf) = reg.get(buf_id)
         {
+            // Arc 6 Stage 2 (Q#FD18): the auto-scroll clamp reckons in
+            // VISIBLE lines. Built from the active window itself, before
+            // the mutable borrow below.
+            let folds = core
+                .windows
+                .get(&active)
+                .and_then(|w| crate::fold_view::map_for_window(&state.fold_registry, w));
             let aw = core.windows.get_mut(&active).expect(
                 "invariant: active_window_id always references a live window in core.windows",
             );
@@ -2867,6 +2905,25 @@ pub fn paint_frame(
                 .text_view
                 .pos_to_display(buf, aw.cursor)
                 .map_or(0, |d| d.row as usize);
+            match folds.as_ref() {
+                // The logical cursor may sit on a hidden line (a shared
+                // fold, or goto-line into one); the row that actually
+                // renders — and so the row to scroll to — is its visible
+                // head (Q#FD16/FD18, framing acceptance 8).
+                Some(map) => {
+                    let anchor = map.visible_head_of(cursor_row);
+                    let top = map.clamp_view_top(aw.view_top);
+                    aw.view_top = if anchor < top {
+                        anchor
+                    } else if inner_rows > 0
+                        && map.visible_rows_between(top, anchor) >= inner_rows as usize
+                    {
+                        map.nth_visible_back(anchor, inner_rows as usize - 1)
+                    } else {
+                        top
+                    };
+                }
+                None => {
                     if cursor_row < aw.view_top {
                         aw.view_top = cursor_row;
                     } else if inner_rows > 0 && cursor_row >= aw.view_top + inner_rows as usize {
@@ -2874,6 +2931,8 @@ pub fn paint_frame(
                     }
                 }
             }
+        }
+    }
 
     // Render every window.
     let registry = core.registry.clone();
@@ -2923,6 +2982,19 @@ pub fn paint_frame(
         let Ok(buf) = reg.get(window.buffer_id) else {
             continue;
         };
+        // Arc 6 Stage 2 (Q#FD12, round-2 F2): ONE visible-line map per
+        // rendered document window, keyed on that window's own buffer and
+        // line offsets. A split may show different buffers with only one
+        // folded, so a per-frame singleton would leak one pane's folds
+        // into the other. `None` when this buffer has no folds — the
+        // unfolded path then paints exactly as before.
+        let folds = crate::fold_view::map_for_window(&state.fold_registry, window);
+        // `view_top` stays a source-line index (Bet B5) but must never
+        // rest on a hidden line: clamp BACKWARD so a fold at the top of
+        // the viewport shows its head (Q#FD18, acceptance 8).
+        if let Some(map) = folds.as_ref() {
+            window.view_top = map.clamp_view_top(window.view_top);
+        }
         let viewport_buffer_start = window.text_view.line_offset(window.view_top).unwrap_or(0);
         // UX gutter (Q#UX2): reserve a left strip for line numbers and
         // shrink+shift the text area into the remainder, so every
@@ -2939,6 +3011,7 @@ pub fn paint_frame(
             cell_origin: CellCoord::new(rect.origin.row, rect.origin.col + gutter_w),
             cell_size: crate::cell::CellSize::new(inner_rows, rect.size.cols - gutter_w),
             gutter_w,
+            folds: folds.as_ref(),
         };
         // Composition (T M2.9): base text_view paints first, then the
         // gutter numbers — before the overlays, so a diagnostic overlay
@@ -2947,24 +3020,52 @@ pub fn paint_frame(
         // overlay in attach order. See [`crate::view::View`].
         window.text_view.render(buf, viewport, grid);
         if gutter_w > 0 {
-            paint_line_number_gutter(grid, window, &rect, inner_rows, gutter_w, &theme);
+            paint_line_number_gutter(
+                grid,
+                window,
+                &rect,
+                inner_rows,
+                gutter_w,
+                folds.as_ref(),
+                &theme,
+            );
         }
         for overlay in &mut window.overlays {
             overlay.render(buf, viewport, grid);
         }
-        paint_local_selection(grid, buf, window, &rect, inner_rows, gutter_w, &theme);
+        paint_local_selection(
+            grid,
+            buf,
+            window,
+            &rect,
+            inner_rows,
+            gutter_w,
+            folds.as_ref(),
+            &theme,
+        );
         // Mode line for this window. Painted last so the line
         // itself is always visible regardless of overlay activity.
         let coord = window
             .text_view
             .pos_to_display(buf, window.cursor)
             .unwrap_or_default();
-        let scroll = format_scroll_indicator(
+        // Arc 6 Stage 2 (Q#FD18): All/Top/Bot/% are reckoned in
+        // VISIBLE-line space — a buffer whose remainder is collapsed
+        // reads "All", not "Top". The cursor's ordinal anchors on its
+        // visible head, since that is the row it renders on.
+        let (ind_top, ind_total, ind_cursor) = match folds.as_ref() {
+            Some(map) => (
+                map.visible_rows_between(0, window.view_top),
+                map.visible_line_count(window.text_view.line_count()),
+                map.visible_rows_between(0, map.visible_head_of(coord.row as usize)),
+            ),
+            None => (
                 window.view_top,
-            inner_rows as usize,
                 window.text_view.line_count(),
                 coord.row as usize,
-        );
+            ),
+        };
+        let scroll = format_scroll_indicator(ind_top, inner_rows as usize, ind_total, ind_cursor);
         // Lock scoped to the summary computation only: the overlay
         // renders above include `DiagnosticView`, which takes this
         // same mutex — holding the guard across the loop deadlocked
@@ -3030,9 +3131,31 @@ pub fn paint_frame(
     let aw = &core.windows[&active];
     let inner_rows = inner_rows(&active_rect);
     let buf = reg.get(aw.buffer_id).ok()?;
-    let disp = aw.text_view.pos_to_display(buf, aw.cursor)?;
+    // Arc 6 Stage 2 (Q#FD16, round-2 F3): a logical cursor on a hidden
-    if (disp.row as usize) < aw.view_top || (disp.row as usize) >= aw.view_top + inner_rows as usize
+    // line renders at its hidden component's head POSITION — the visible
-    {
+    // head row *and* that head's end-of-content column, i.e. exactly
+    // where Stage 1 moves point on a fold-at-cursor. Row-only clamping
+    // would leave the column unspecified; resolving through the merged
+    // component (rather than the innermost containing fold) also keeps a
+    // crossing overlap from landing on another hidden position.
+    let folds = crate::fold_view::map_for_window(&state.fold_registry, aw);
+    let cursor = match folds.as_ref() {
+        Some(map) => map.visible_position(aw.text_view.line_at_offset(aw.cursor), aw.cursor),
+        None => aw.cursor,
+    };
+    let disp = aw.text_view.pos_to_display(buf, cursor)?;
+    let row_offset = match folds.as_ref() {
+        Some(map) => {
+            let top = map.clamp_view_top(aw.view_top);
+            let row = disp.row as usize;
+            if row < top {
+                return None;
+            }
+            map.visible_rows_between(top, row)
+        }
+        None => (disp.row as usize).checked_sub(aw.view_top)?,
+    };
+    if row_offset >= inner_rows as usize {
         return None;
     }
     // UX gutter: the terminal caret sits in the text area, past the
@@ -3041,7 +3164,7 @@ pub fn paint_frame(
         let w = aw.gutter_width();
         if w >= active_rect.size.cols { 0 } else { w }
     };
-    let grid_row = active_rect.origin.row + (disp.row - aw.view_top as u32);
+    let grid_row = active_rect.origin.row + u32::try_from(row_offset).ok()?;
     let max_col = active_rect.origin.col + active_rect.size.cols.saturating_sub(1);
     let grid_col = (active_rect.origin.col + gutter_w + disp.col).min(max_col);
     Some(CellCoord::new(grid_row, grid_col))
@@ -3180,13 +3303,23 @@ fn paint_line_number_gutter(
     rect: &crate::window::Rect,
     inner_rows: u32,
     gutter_w: u32,
+    // Arc 6 Stage 2: this window's collapsed regions, or `None` when it
+    // has no folds (then every line below is the pre-folding walk).
+    folds: Option<&crate::fold_view::VisibleLineMap>,
     theme: &crate::highlight::Theme,
 ) {
     let line_count = window.text_view.line_count();
     // Relative/Hybrid measure distance from the cursor's buffer line;
     // Absolute ignores it. Computed once per frame (the gutter repaints on
     // cursor motion, so this stays current).
+    //
+    // Arc 6 Stage 2 (Q#FD14): the anchor is the cursor's **visible head**
+    // — a shared fold (or goto-line) can leave the logical cursor on a
+    // hidden line, and the distance must be measured from the row the
+    // caret actually renders on. With no folds this is `cursor_line`
+    // verbatim, so the unfolded gutter is unchanged.
     let cursor_line = window.text_view.line_at_offset(window.cursor);
+    let anchor = folds.map_or(cursor_line, |map| map.visible_head_of(cursor_line));
     // Themes Q#TH5: a set `ui.gutter` face owns the strip within its
     // {fg} mask; unset keeps the dim Indexed(8).
     let style = theme.face("ui.gutter").map_or(
@@ -3202,6 +3335,9 @@ fn paint_line_number_gutter(
     // The number's rightmost digit sits at `field - 1`; the last gutter
     // cell (`gutter_w - 1`) is a trailing pad separating it from the code.
     let field = gutter_w.saturating_sub(1);
+    // Row `r` shows the `r`-th VISIBLE line at or after `view_top`, the
+    // same walk `TextView::render` performs (Q#FD13/FD14).
+    let mut buffer_line = folds.map_or(window.view_top, |map| map.visible_head_of(window.view_top));
     for r in 0..inner_rows {
         let grid_row = rect.origin.row + r;
         // Blank + style the whole strip first, so a number that shrank a
@@ -3212,16 +3348,43 @@ fn paint_line_number_gutter(
             cell.style = style;
             cell.attachment = None;
         }
-        let buffer_line = window.view_top + r as usize;
         if buffer_line >= line_count {
             continue; // past end-of-buffer: blank gutter
         }
+        let this_line = buffer_line;
+        buffer_line = folds.map_or(this_line + 1, |map| map.next_visible(this_line));
+
+        // Fold marker (Q#FD20, round-1 F3): the col-0 sign cell only
+        // exists when a gutter does, so the glyph is conditional on it —
+        // with line numbers off the content-area ellipsis is the sole
+        // indicator. Painted here, before the overlays: `DiagnosticView`
+        // writes the same cell later in the frame, so a diagnostic
+        // clamped onto this head wins (an error inside the collapsed
+        // region is higher-signal than "this is collapsed").
+        if folds.is_some_and(|map| map.is_head(this_line)) {
+            grid.at(CellCoord::new(grid_row, rect.origin.col)).glyph =
+                crate::cell::Glyph::Char(FOLD_GUTTER_GLYPH);
+        }
+
         // The mode picks the number: absolute (`line+1`), relative
         // distance, or hybrid (absolute on the cursor line, else relative).
         // Written right-aligned, rightmost digit first, alloc-free.
         // `field >= digits(line_count)` by construction, so the leftmost
         // digit always leaves at least a leading pad cell.
-        let Some(mut val) = window.line_numbers.number_for(buffer_line, cursor_line) else {
+        //
+        // Arc 6 Stage 2 (Q#FD14): with folds present, Relative/Hybrid
+        // distance is counted in VISIBLE lines across the collapse;
+        // Absolute keeps the raw `line + 1` (hidden numbers simply do not
+        // appear, so the column jumps from the head's number to the first
+        // post-fold number). Without folds this is `number_for` verbatim.
+        let number = match (folds, window.line_numbers) {
+            (Some(map), LineNumberMode::Relative) => Some(map.visible_distance(anchor, this_line)),
+            (Some(map), LineNumberMode::Hybrid) if this_line != anchor => {
+                Some(map.visible_distance(anchor, this_line))
+            }
+            _ => window.line_numbers.number_for(this_line, anchor),
+        };
+        let Some(mut val) = number else {
             continue;
         };
         let mut col = field;
@@ -3238,6 +3401,10 @@ fn paint_line_number_gutter(
     }
 }
 
+#[allow(
+    clippy::too_many_arguments,
+    reason = "one window's already-resolved paint geometry; mirrors paint_selection_in_window"
+)]
 fn paint_local_selection(
     grid: &mut crate::cell::CellGrid<'_>,
     buf: &crate::buffer::Buffer,
@@ -3248,11 +3415,24 @@ fn paint_local_selection(
     // text-relative display column shifted right by this (Q#UX2). 0 when
     // the gutter is off, so this is a no-op then.
     gutter_w: u32,
+    // Arc 6 Stage 2: this window's collapsed regions, or `None`.
+    folds: Option<&crate::fold_view::VisibleLineMap>,
     theme: &crate::highlight::Theme,
 ) {
     let Some((sel_start, sel_end)) = window.region() else {
         return;
     };
+    // Arc 6 Stage 2 (Q#FD16): each ENDPOINT on a hidden line projects to
+    // its component's head position; hidden interior cells simply have no
+    // row and drop. The visible portion then paints on the visible head
+    // row and the visible tail rows, contiguous on screen.
+    let (sel_start, sel_end) = match folds {
+        Some(map) => (
+            map.visible_position(window.text_view.line_at_offset(sel_start), sel_start),
+            map.visible_position(window.text_view.line_at_offset(sel_end), sel_end),
+        ),
+        None => (sel_start, sel_end),
+    };
     // Themes Q#TH5: the selection is a wash — a set `ui.selection`
     // face replaces the default overlay wholesale within its {bg}
     // mask (an all-default face disables the wash; out-of-mask
@@ -3272,9 +3452,11 @@ fn paint_local_selection(
     }
     let text_cols = rect.size.cols.saturating_sub(gutter_w);
 
-    let first_row = window.view_top;
+    // Row `r` shows the `r`-th VISIBLE line at or after `view_top`.
-    let last_row = first_row.saturating_add(inner_rows as usize);
+    let mut next_line = folds.map_or(window.view_top, |map| map.visible_head_of(window.view_top));
-    for display_row in first_row..last_row {
+    for row_offset in 0..inner_rows {
+        let display_row = next_line;
+        next_line = folds.map_or(display_row + 1, |map| map.next_visible(display_row));
         let Some(line_start) = window.text_view.line_offset(display_row) else {
             continue;
         };
@@ -3298,7 +3480,6 @@ fn paint_local_selection(
             continue;
         }
 
-        let row_offset = display_row.saturating_sub(first_row) as u32;
         let start_col = start_coord.col.min(text_cols);
         let end_col = end_coord.col.min(text_cols);
         if start_col >= end_col {
@@ -4028,6 +4209,7 @@ mod tests {
             &rect,
             rows,
             4,
+            None,
             &crate::highlight::Theme::empty(),
         );
 
@@ -6672,6 +6854,7 @@ mod tests {
             cell_origin: rect.origin,
             cell_size: CellSize::new(rect.size.rows, rect.size.cols),
             gutter_w: 0,
+            folds: None,
         };
         let mut grid = CellGrid {
             cells: &mut backing,
@@ -6806,6 +6989,7 @@ mod tests {
                 cell_origin: CellCoord::new(0, 0),
                 cell_size: CellSize::new(24, 80),
                 gutter_w: 0,
+                folds: None,
             };
 
             // Two no-op overlays: probe the dispatch cost only.

src/editor_core.rs

@@ -386,6 +386,8 @@ impl EditorCore {
             FrontendView {
                 layout: Layout::single(id),
                 active: id,
+                // LOCAL is the in-process grid editor (Q#FD21).
+                fold_projection: true,
             },
         );
         Self {
@@ -534,6 +536,51 @@ impl EditorCore {
         self.windows.get_mut(&win_id)
     }
 
+    /// Whether the **acting** frontend's display collapses folds (Arc 6
+    /// Stage 2, Q#FD21).
+    ///
+    /// The gate on every command/event-time visible-line reckoning —
+    /// motion, paging, wheel, the click inverse, the auto-scroll clamp.
+    /// A `semantic_render` (GPU) session still displays every source
+    /// line until Stage 3, so with this `false` those sites keep their
+    /// raw-line behavior and its cursor never skips a line it is showing
+    /// (even while a grid session folds the same shared buffer).
+    #[must_use]
+    pub fn fold_projection_active(&self) -> bool {
+        self.active_view().fold_projection
+    }
+
+    /// The visible-line map for `win_id`'s buffer, or `None` when the
+    /// acting frontend does not project folds, `win_id` is unknown, or
+    /// that buffer has no folds (Q#FD12).
+    ///
+    /// The two axes are deliberately separate (round-3 F1): the **acting
+    /// frontend** supplies the projection policy, while the
+    /// **operation's target window** supplies the buffer and line
+    /// offsets. Motion, paging, and auto-scroll target the active
+    /// window; the click inverse and wheel scrolling name an explicit
+    /// `win_id` — a wheel event over an inactive pane does not activate
+    /// it, so deriving the active window's map there would project one
+    /// buffer's folds onto another.
+    #[must_use]
+    pub fn fold_map_for_window(
+        &self,
+        win_id: WindowId,
+    ) -> Option<crate::fold_view::VisibleLineMap> {
+        if !self.fold_projection_active() {
+            return None;
+        }
+        let window = self.windows.get(&win_id)?;
+        crate::fold_view::map_for_window(&self.fold_registry, window)
+    }
+
+    /// [`Self::fold_map_for_window`] for the active window — the target
+    /// of motion, paging, and the auto-scroll clamp.
+    #[must_use]
+    pub fn fold_map_active(&self) -> Option<crate::fold_view::VisibleLineMap> {
+        self.fold_map_for_window(self.active_window_id())
+    }
+
     /// T M10.8 — register a `FrontendView` for `fid`. Called by the
     /// daemon on attach (Day 3 dispatcher work). Day 2's fallback
     /// path makes this optional; Day 3 makes it required.
@@ -664,9 +711,21 @@ impl EditorCore {
     /// clamped to the buffer's line count (Arc 3 Q#PS1 — desktop
     /// restore). A file that shrank since the desktop was saved can't
     /// scroll past its end.
+    /// Arc 6 Stage 2 (Q#FD12/Q#FD18, round-4 F3): `view_top` stays a
+    /// source-line index but must never *name* a hidden line, so a
+    /// fold-projecting frontend also clamps **backward** to the visible
+    /// head here. The render pass repairs a hidden `view_top` too, but
+    /// only at the next frame — until then [`Self::view_top`] would hand
+    /// out a collapsed line and command/event reckoning would start from
+    /// a non-visible origin. This setter is the contract's home (it is
+    /// what `saveplace` and `pmacs.editor.set_view_top` call), so the
+    /// invariant is established here rather than repaired downstream.
     pub fn set_view_top(&mut self, top: usize) {
         let lines = self.active_window().text_view.line_count();
         let clamped = top.min(lines.saturating_sub(1));
+        let clamped = self
+            .fold_map_active()
+            .map_or(clamped, |map| map.clamp_view_top(clamped));
         self.active_window_mut().view_top = clamped;
     }
 
@@ -1267,6 +1326,16 @@ impl EditorCore {
     ///
     /// Returns a stringified error on buffer or view failure.
     pub fn apply_active_edit(&mut self, op: EditOp<'_>) -> Result<Edit, String> {
+        // Arc 6 Stage 2 (Q#FD19): ONE pre-edit unfold funnel for every
+        // local point-anchored edit. This subsumes the six `dispatch_key`
+        // primitives' individual calls (retired) and widens the behavior
+        // to **yank** and **query-replace**, which reach the buffer
+        // through here rather than through those primitives — both place
+        // point at the edit site first, so keying on the active point
+        // covers them exactly. `apply_active_edit` is never the
+        // remote-apply path, so this funnel is inherently local: a remote
+        // peer's edit inside my fold must not unfold it (Stage 3).
+        self.unfold_before_point_edit();
         let buffer_id = self.active_buffer_id();
         // Scope the registry borrow: the origin translation below needs
         // `&mut self` after the views have been notified.
@@ -1544,27 +1613,48 @@ impl EditorCore {
     }
 
     /// Move the cursor up one line, preserving display column.
+    ///
+    /// Arc 6 Stage 2 (Q#FD17, ruled: include): on a fold-projecting
+    /// frontend this steps to the previous **visible** line, so a
+    /// collapsed region is one motion step and the cursor never comes to
+    /// rest hidden. Motion that *begins* from a hidden logical cursor (a
+    /// shared fold, or goto-line into one) first normalizes to the
+    /// visible head. Scoped by Q#FD21 — a semantic frontend keeps
+    /// raw-line motion until Stage 3.
     pub fn move_up(&mut self) {
+        let folds = self.fold_map_active();
+        // Normalize FIRST and as a real mutation (round-4 F2): a hidden
+        // logical cursor moves to its component's head POSITION — head
+        // row *and* head end-of-content column — before any step is
+        // computed. Deriving the goal column from the hidden line would
+        // carry a column the head may not even have, and returning early
+        // at a buffer boundary (a fold headed on line 0) would leave the
+        // cursor hidden entirely.
+        self.normalize_cursor_to_visible(folds.as_ref());
         let id = self.active_buffer_id();
         let cursor = self.active_window().cursor;
         let goal_col = self.active_window().goal_col;
         let result = {
             let reg = self.registry.borrow();
             let Ok(buffer) = reg.get(id) else { return };
-            let coord = self
+            let aw = self.active_window();
-                .active_window()
+            let coord = aw
                 .text_view
                 .pos_to_display(buffer, cursor)
                 .unwrap_or_default();
-            if coord.row == 0 {
+            let from_row = coord.row as usize;
+            if from_row == 0 {
                 return;
             }
+            let target_row = folds
+                .as_ref()
+                .map_or(from_row - 1, |map| map.prev_visible(from_row));
             let goal = goal_col.unwrap_or(coord.col);
-            let target = DisplayCoord::new(coord.row - 1, goal);
+            let Ok(target_row) = u32::try_from(target_row) else {
-            let new_pos = self
+                return;
-                .active_window()
+            };
-                .text_view
+            let target = DisplayCoord::new(target_row, goal);
-                .display_to_pos(buffer, target);
+            let new_pos = aw.text_view.display_to_pos(buffer, target);
             (goal, new_pos)
         };
         let (goal, new_pos) = result;
@@ -1576,28 +1666,34 @@ impl EditorCore {
     }
 
     /// Move the cursor down one line, preserving display column.
+    /// Visible-line stepping mirrors [`Self::move_up`] (Q#FD17/FD21).
     pub fn move_down(&mut self) {
+        let folds = self.fold_map_active();
+        self.normalize_cursor_to_visible(folds.as_ref());
         let id = self.active_buffer_id();
         let cursor = self.active_window().cursor;
         let goal_col = self.active_window().goal_col;
         let result = {
             let reg = self.registry.borrow();
             let Ok(buffer) = reg.get(id) else { return };
-            let coord = self
+            let aw = self.active_window();
-                .active_window()
+            let coord = aw
                 .text_view
                 .pos_to_display(buffer, cursor)
                 .unwrap_or_default();
-            let next_row = coord.row + 1;
+            let from_row = coord.row as usize;
-            if (next_row as usize) >= self.active_window().text_view.line_count() {
+            let next_row = folds
+                .as_ref()
+                .map_or(from_row + 1, |map| map.next_visible(from_row));
+            if next_row >= aw.text_view.line_count() {
                 return;
             }
             let goal = goal_col.unwrap_or(coord.col);
+            let Ok(next_row) = u32::try_from(next_row) else {
+                return;
+            };
             let target = DisplayCoord::new(next_row, goal);
-            let new_pos = self
+            let new_pos = aw.text_view.display_to_pos(buffer, target);
-                .active_window()
-                .text_view
-                .display_to_pos(buffer, target);
             (goal, new_pos)
         };
         let (goal, new_pos) = result;
@@ -1746,6 +1842,12 @@ impl EditorCore {
     /// of context); falls back to a sane default before the first
     /// frame has rendered.
     pub fn move_page_down(&mut self) {
+        // Arc 6 Stage 2 (Q#FD18/FD21): a screenful is a screenful of
+        // VISIBLE lines, and `view_top` never lands hidden. Paging shares
+        // vertical motion's hidden-cursor normalization (round-4 F2) —
+        // the goal column must come from the head, not the hidden line.
+        let folds = self.fold_map_active();
+        self.normalize_cursor_to_visible(folds.as_ref());
         let step = self.page_step();
         let cursor = self.active_window().cursor;
         let view_top = self.active_window().view_top;
@@ -1758,12 +1860,25 @@ impl EditorCore {
                 .text_view
                 .pos_to_display(buffer, cursor)
                 .unwrap_or_default();
-            let max_line = aw.text_view.line_count().saturating_sub(1) as u32;
+            let max_line = aw.text_view.line_count().saturating_sub(1);
             let goal_col = aw.goal_col.unwrap_or(coord.col);
-            let target_row = (coord.row + step).min(max_line);
+            let (target_row, new_top) = match folds.as_ref() {
+                Some(map) => (
+                    map.nth_visible_from(coord.row as usize, step as usize)
+                        .min(map.visible_head_of(max_line)),
+                    map.nth_visible_from(view_top, step as usize),
+                ),
+                None => (
+                    (coord.row as usize + step as usize).min(max_line),
+                    view_top.saturating_add(step as usize),
+                ),
+            };
+            let Ok(target_row) = u32::try_from(target_row) else {
+                return;
+            };
             let target = DisplayCoord::new(target_row, goal_col);
             let new_pos = aw.text_view.display_to_pos(buffer, target);
-            (goal_col, new_pos, view_top.saturating_add(step as usize))
+            (goal_col, new_pos, new_top)
         };
         let (goal, new_pos, new_top) = result;
         let aw = self.active_window_mut();
@@ -1774,12 +1889,17 @@ impl EditorCore {
         // Also nudge view_top; render's scroll-into-view will clamp
         // and align further if needed.
         let max_top = aw.text_view.line_count().saturating_sub(1);
-        aw.view_top = new_top.min(max_top);
+        let clamped = new_top.min(max_top);
+        aw.view_top = folds
+            .as_ref()
+            .map_or(clamped, |map| map.clamp_view_top(clamped));
     }
 
     /// Move the cursor up by approximately one screenful. Mirror of
     /// [`Self::move_page_down`].
     pub fn move_page_up(&mut self) {
+        let folds = self.fold_map_active();
+        self.normalize_cursor_to_visible(folds.as_ref());
         let step = self.page_step();
         let cursor = self.active_window().cursor;
         let view_top = self.active_window().view_top;
@@ -1793,10 +1913,22 @@ impl EditorCore {
                 .pos_to_display(buffer, cursor)
                 .unwrap_or_default();
             let goal_col = aw.goal_col.unwrap_or(coord.col);
-            let target_row = coord.row.saturating_sub(step);
+            let (target_row, new_top) = match folds.as_ref() {
+                Some(map) => (
+                    map.nth_visible_back(coord.row as usize, step as usize),
+                    map.nth_visible_back(view_top, step as usize),
+                ),
+                None => (
+                    (coord.row as usize).saturating_sub(step as usize),
+                    view_top.saturating_sub(step as usize),
+                ),
+            };
+            let Ok(target_row) = u32::try_from(target_row) else {
+                return;
+            };
             let target = DisplayCoord::new(target_row, goal_col);
             let new_pos = aw.text_view.display_to_pos(buffer, target);
-            (goal_col, new_pos, view_top.saturating_sub(step as usize))
+            (goal_col, new_pos, new_top)
         };
         let (goal, new_pos, new_top) = result;
         let aw = self.active_window_mut();
@@ -1838,15 +1970,49 @@ impl EditorCore {
         aw.goal_col = None;
     }
 
-    /// Dispatch-layer pre-edit unfold (Arc 6, Q#FD5). Before a
+    /// Pre-edit unfold (Arc 6, Q#FD5 / Stage 2 Q#FD19). Before a local
-    /// command-path point-anchored edit (the six primitives below),
+    /// point-anchored edit, unfold every fold containing the active point
-    /// unfold every fold containing the active point so a self-insert or
+    /// so an edit inside a collapsed region reveals it rather than
-    /// delete inside a collapsed region reveals it rather than landing
+    /// landing invisibly. Keyed on the authenticated source frontend's
-    /// invisibly. Keyed on the authenticated source frontend's active
+    /// active point (`active_window().cursor`), not the transport. A
-    /// point (this is `active_window().cursor`), not the transport. A
+    /// no-op when the buffer has no folds.
-    /// no-op when the buffer has no folds. Interactive Lua-command edits
+    ///
-    /// (yank/query-replace/comment) reach the buffer through a different
+    /// **Stage 2 widening:** Stage 1 called this from each of the six
-    /// path and are a named Stage 2 widening; CRDT-origin is Stage 3.
+    /// `dispatch_key` edit primitives. It now runs once at the top of
+    /// [`Self::apply_active_edit`] — the single funnel those primitives
+    /// (and yank, and query-replace) all pass through. Interactive
+    /// Lua-mutator edits (comment-toggle, yank-pop) take a *different*
+    /// path and are hooked at `run_buffer_edit` in the Lua bindings; the
+    /// remote/optimistic-CRDT apply path is deliberately excluded
+    /// (Stage 3), as is undo/redo (deferred).
+    /// Move a hidden logical cursor to its component's visible head
+    /// **position** — the head row *and* that head's end-of-content
+    /// column, i.e. exactly where Stage 1 moves point on a fold-at-cursor
+    /// (Q#FD16/Q#FD17, round-4 F2).
+    ///
+    /// A cursor can be hidden without this frontend ever having moved it
+    /// there: another frontend folds through the shared store, or
+    /// goto-line targets a line inside a collapse. Vertical motion and
+    /// paging normalize through here *before* computing their step, so
+    /// the goal column is the head's — never a column of a line that
+    /// renders no row — and so a step that turns out to be a no-op at a
+    /// buffer boundary still leaves the cursor visible.
+    ///
+    /// The jump is discontinuous, so the sticky goal column is dropped
+    /// (the click/goto convention), and only when the cursor actually
+    /// was hidden: ordinary motion keeps its sticky column untouched.
+    fn normalize_cursor_to_visible(&mut self, folds: Option<&crate::fold_view::VisibleLineMap>) {
+        let Some(map) = folds else { return };
+        let aw = self.active_window();
+        let line = aw.text_view.line_at_offset(aw.cursor);
+        let projected = map.visible_position(line, aw.cursor);
+        if projected != aw.cursor {
+            let aw = self.active_window_mut();
+            aw.cursor = projected;
+            aw.goal_col = None;
+        }
+    }
+
     fn unfold_before_point_edit(&self) {
         let id = self.active_buffer_id();
         let point = self.active_window().cursor;
@@ -1858,7 +2024,6 @@ impl EditorCore {
     /// line and returns `false`, and callers must not mutate dependent
     /// state (e.g. selection anchors) on a failed insert (Q#AI9).
     pub fn insert_char(&mut self, ch: char) -> bool {
-        self.unfold_before_point_edit();
         self.active_window_mut().goal_col = None;
         let mut buf = [0u8; 4];
         let s = ch.encode_utf8(&mut buf);
@@ -1894,7 +2059,6 @@ impl EditorCore {
     /// delegates to [`Self::insert_char`] (a plain insert). The cursor
     /// lands just past the inserted bytes and any selection is cleared.
     pub fn insert_char_over_region(&mut self, ch: char) {
-        self.unfold_before_point_edit();
         let Some((lo, hi)) = self.active_region() else {
             // Q#AI9: an empty selection (anchor == cursor) reports no
             // region yet stays armed — the insert moves the cursor off
@@ -1936,7 +2100,6 @@ impl EditorCore {
 
     /// Delete the codepoint immediately before the cursor.
     pub fn backspace(&mut self) {
-        self.unfold_before_point_edit();
         self.active_window_mut().goal_col = None;
         let cursor = self.active_window().cursor;
         if cursor == 0 {
@@ -1958,7 +2121,6 @@ impl EditorCore {
 
     /// Delete the codepoint at the cursor (forward delete).
     pub fn delete_forward(&mut self) {
-        self.unfold_before_point_edit();
         self.active_window_mut().goal_col = None;
         let cursor = self.active_window().cursor;
         let id = self.active_buffer_id();
@@ -1982,7 +2144,6 @@ impl EditorCore {
     /// between the cursor and where [`Self::move_word_left`] would
     /// land.
     pub fn delete_word_backward(&mut self) {
-        self.unfold_before_point_edit();
         self.active_window_mut().goal_col = None;
         let cursor = self.active_window().cursor;
         if cursor == 0 {
@@ -2010,7 +2171,6 @@ impl EditorCore {
     /// [`Self::delete_forward`] over the gap from the cursor to where
     /// [`Self::move_word_right`] would land.
     pub fn delete_word_forward(&mut self) {
-        self.unfold_before_point_edit();
         self.active_window_mut().goal_col = None;
         let cursor = self.active_window().cursor;
         let id = self.active_buffer_id();
@@ -3348,6 +3508,7 @@ mod tests {
             FrontendView {
                 layout: Layout::single(win_id),
                 active: win_id,
+                fold_projection: true,
             },
         );
         win_id

src/fold_view.rs

@@ -0,0 +1,466 @@
+// fold_view.rs --- The visible-line map (Arc 6, Stage 2).
+
+//! The source-line ↔ display-row projection that folding introduces.
+//!
+//! Before folding, every grid consumer assumed `display_row =
+//! source_line − view_top` — an identity map baked into the text walk,
+//! the gutter, every overlay, the caret, both selection painters, the
+//! mode-line indicator, and the click/scroll/motion inverses.
+//! [`DisplayCoord`](crate::view::DisplayCoord) anticipated a
+//! non-identity map "once virtual lines, wrapping, and inline
+//! expansions appear"; **folding is the first**.
+//!
+//! This module is that map — **one derivation/query primitive**
+//! (`docs/folding-stage2-framing.md` Q#FD12), derived from
+//! [`crate::fold::FoldRegistry::folds`] plus the buffer's line offsets
+//! and **never stored**: the byte-range store in [`crate::fold`] stays
+//! the single source of truth. Instances are short-lived and built
+//! **per rendered window** and **per command/event operation**, never
+//! once per frame — a frame paints several windows that may show
+//! different buffers, so a per-frame singleton would leak one pane's
+//! folds into another's (framing round-2 F2).
+//!
+//! # Hidden components, not folds
+//!
+//! The unit here is not a fold. [`crate::fold::FoldStore::insert`]
+//! accepts any normalized range, so folds may nest, share a head line,
+//! or **cross**: with fold `A` hiding lines 1–3 and fold `B` headed on
+//! line 2 hiding lines 3–5, a point on line 5 is directly inside only
+//! `B` — yet `B`'s own head is hidden by `A`, so projecting to `B`'s
+//! `range.start` would land on another *hidden* position (round-3 F2).
+//!
+//! The derivation therefore unions overlapping **or adjacent** hidden
+//! line intervals into sorted, non-overlapping **hidden components**.
+//! Adjacent intervals merge because the later fold's head is hidden by
+//! the earlier one, so it can never render. Each component keeps the one
+//! visible line immediately before it (`head_line`) and that line's exact
+//! end-of-content byte (`head_position` — the fold `range.start` Stage 1
+//! already moves point to). Resolving through the component is
+//! equivalent to repeatedly projecting a hidden fold head until it is
+//! visible, and so covers nesting, shared heads, and crossing overlap
+//! alike.
+
+use pmacs_protocol::ByteRange;
+
+use crate::fold::FoldRegistry;
+use crate::rope::Position;
+use crate::window::Window;
+
+/// The visible-line map for one **window's** buffer, or `None` when that
+/// buffer has no folds.
+///
+/// The single construction rule shared by the render path and
+/// `EditorCore` (Q#FD12): keyed on *this* window's `buffer_id` and its
+/// own [`TextView`](crate::text_view::TextView) line offsets — never the
+/// active buffer's — so a split showing two buffers gets two independent
+/// maps and neither leaks into the other (round-2 F2). Returning `None`
+/// rather than an empty map keeps the unfolded path byte-identical.
+#[must_use]
+pub fn map_for_window(registry: &FoldRegistry, window: &Window) -> Option<VisibleLineMap> {
+    let folds = registry.folds(window.buffer_id);
+    if folds.is_empty() {
+        return None;
+    }
+    Some(VisibleLineMap::build(&folds, |off| {
+        window.text_view.line_at_offset(off)
+    }))
+}
+
+/// A maximal run of consecutive hidden source lines, plus the one
+/// visible line that heads it.
+///
+/// `first_hidden >= 1` always: a component's `head_line` is
+/// `first_hidden - 1`, and a fold's head line is the line *above* its
+/// first hidden line, so line 0 can never be hidden.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+struct HiddenComponent {
+    /// First hidden source line (inclusive).
+    first_hidden: usize,
+    /// Last hidden source line (inclusive).
+    last_hidden: usize,
+    /// End-of-content byte of `head_line` — the `ByteRange::start` of
+    /// the earliest fold participating in this component, which is
+    /// exactly where Stage 1 moves point on a fold-at-cursor.
+    head_position: Position,
+}
+
+impl HiddenComponent {
+    /// The one visible line immediately above this component.
+    const fn head_line(&self) -> usize {
+        self.first_hidden - 1
+    }
+}
+
+/// A buffer's collapsed regions, projected into line space.
+///
+/// Derived from a fold list and a byte→line lookup; cheap enough to
+/// rebuild per window per frame (**Bet B4**: `O(folds)` with one binary
+/// search into the caller's existing line-offset table per fold, and
+/// folds are `O(top-level blocks)`).
+///
+/// An empty map (`is_identity`) means "no folds" — callers pass `None`
+/// rather than an empty map so the unfolded path stays byte-identical.
+#[derive(Clone, Debug, Default, Eq, PartialEq)]
+pub struct VisibleLineMap {
+    /// Sorted by `first_hidden`, non-overlapping, and separated by at
+    /// least one visible line (adjacency is merged away at build time).
+    components: Vec<HiddenComponent>,
+}
+
+impl VisibleLineMap {
+    /// Derive the map from a buffer's folds.
+    ///
+    /// `line_at_offset` is the caller's own line-offset lookup (the
+    /// rendering window's [`TextView`](crate::text_view::TextView), the
+    /// only table guaranteed to agree with the rows being painted). A
+    /// fold's stored range is `[end of head line, end of last hidden
+    /// line]`, so `head_line = line_at_offset(start)` and `last_hidden =
+    /// line_at_offset(end)`; a fold that no longer spans a whole line
+    /// (mid-edit drift) contributes nothing.
+    #[must_use]
+    pub fn build<F>(folds: &[ByteRange], line_at_offset: F) -> Self
+    where
+        F: Fn(Position) -> usize,
+    {
+        let mut raw: Vec<HiddenComponent> = folds
+            .iter()
+            .filter_map(|f| {
+                let head_line = line_at_offset(f.start);
+                let last_hidden = line_at_offset(f.end);
+                (last_hidden > head_line).then_some(HiddenComponent {
+                    first_hidden: head_line + 1,
+                    last_hidden,
+                    head_position: f.start,
+                })
+            })
+            .collect();
+        raw.sort_by(|a, b| {
+            a.first_hidden
+                .cmp(&b.first_hidden)
+                .then(a.last_hidden.cmp(&b.last_hidden))
+        });
+        let mut components: Vec<HiddenComponent> = Vec::with_capacity(raw.len());
+        for c in raw {
+            match components.last_mut() {
+                // Overlapping OR adjacent: `c`'s head line is itself
+                // hidden by `prev`, so it can never render — the merged
+                // component keeps `prev`'s (visible) head.
+                Some(prev) if c.first_hidden <= prev.last_hidden + 1 => {
+                    prev.last_hidden = prev.last_hidden.max(c.last_hidden);
+                }
+                _ => components.push(c),
+            }
+        }
+        Self { components }
+    }
+
+    /// Whether this map hides nothing — the identity projection.
+    #[must_use]
+    pub fn is_identity(&self) -> bool {
+        self.components.is_empty()
+    }
+
+    /// The component hiding `line`, if any.
+    fn component_of(&self, line: usize) -> Option<&HiddenComponent> {
+        let after = self.components.partition_point(|c| c.first_hidden <= line);
+        let c = self.components.get(after.checked_sub(1)?)?;
+        (line <= c.last_hidden).then_some(c)
+    }
+
+    /// Whether `line` is collapsed away and renders no row.
+    #[must_use]
+    pub fn is_hidden(&self, line: usize) -> bool {
+        self.component_of(line).is_some()
+    }
+
+    /// Whether `line` is the visible head of a collapsed region — the
+    /// row that carries the ellipsis and the gutter fold glyph.
+    #[must_use]
+    pub fn is_head(&self, line: usize) -> bool {
+        self.components
+            .binary_search_by(|c| c.first_hidden.cmp(&(line + 1)))
+            .is_ok()
+    }
+
+    /// The **outermost visible head** of `line`: for a hidden line, its
+    /// component's head line; for a visible line, itself.
+    ///
+    /// The **row-only** clamp — diagnostic signs, the relative-number
+    /// cursor anchor, and the backward `view_top` clamp. Positions that
+    /// carry a column use [`Self::visible_position`] instead.
+    #[must_use]
+    pub fn visible_head_of(&self, line: usize) -> usize {
+        self.component_of(line)
+            .map_or(line, HiddenComponent::head_line)
+    }
+
+    /// The **position** projection of a byte on `line`: for a hidden
+    /// line, its component's `head_position` (the head line's
+    /// end-of-content byte); for a visible line, `pos` unchanged.
+    ///
+    /// Used wherever a clamp carries a column — the local caret, peer
+    /// cursors, and selection endpoints — so a hidden point lands at the
+    /// head's end of content rather than at an arbitrary column on the
+    /// head (round-2 F3) or at a still-hidden crossing fold's start
+    /// (round-3 F2).
+    #[must_use]
+    pub fn visible_position(&self, line: usize, pos: Position) -> Position {
+        self.component_of(line).map_or(pos, |c| c.head_position)
+    }
+
+    /// Clamp a candidate `view_top` **backward** to a visible line, so a
+    /// fold at the top of the viewport shows its head rather than being
+    /// skipped past (framing acceptance 8).
+    #[must_use]
+    pub fn clamp_view_top(&self, line: usize) -> usize {
+        self.visible_head_of(line)
+    }
+
+    /// The next visible line strictly after `line`, skipping whole
+    /// collapsed regions. May exceed the buffer's line count; callers
+    /// bound it themselves.
+    #[must_use]
+    pub fn next_visible(&self, line: usize) -> usize {
+        let next = line + 1;
+        self.component_of(next)
+            .map_or(next, |c| c.last_hidden.saturating_add(1))
+    }
+
+    /// The previous visible line strictly before `line`, or `0` when
+    /// `line` is already the first line.
+    #[must_use]
+    pub fn prev_visible(&self, line: usize) -> usize {
+        match line.checked_sub(1) {
+            Some(prev) => self.visible_head_of(prev),
+            None => 0,
+        }
+    }
+
+    /// Number of visible lines in the half-open range `[from, to)`;
+    /// `0` when `to <= from`.
+    ///
+    /// This is the framing's `visible_between` — exposed unsigned and
+    /// half-open (plus the symmetric [`Self::visible_distance`]) because
+    /// no consumer reads the sign: row offsets always measure forward
+    /// from `view_top`, and relative line numbers want a magnitude.
+    #[must_use]
+    pub fn visible_rows_between(&self, from: usize, to: usize) -> usize {
+        if to <= from {
+            return 0;
+        }
+        (to - from) - self.hidden_in(from, to)
+    }
+
+    /// Visible-line distance between `a` and `b`, either order — the
+    /// relative/hybrid gutter number measured across collapses.
+    #[must_use]
+    pub fn visible_distance(&self, a: usize, b: usize) -> usize {
+        if a <= b {
+            self.visible_rows_between(a, b)
+        } else {
+            self.visible_rows_between(b, a)
+        }
+    }
+
+    /// Hidden lines within the half-open range `[from, to)`.
+    fn hidden_in(&self, from: usize, to: usize) -> usize {
+        self.components
+            .iter()
+            .filter(|c| c.first_hidden < to && c.last_hidden >= from)
+            .map(|c| {
+                // `lo <= hi` holds under the filter, so this cannot
+                // underflow.
+                let lo = c.first_hidden.max(from);
+                let hi = c.last_hidden.min(to - 1);
+                hi + 1 - lo
+            })
+            .sum()
+    }
+
+    /// Total visible lines in a buffer of `total_lines` source lines —
+    /// the denominator the mode-line scroll indicator reckons in.
+    #[must_use]
+    pub fn visible_line_count(&self, total_lines: usize) -> usize {
+        total_lines - self.hidden_in(0, total_lines).min(total_lines)
+    }
+
+    /// The line `n` visible steps forward from `from` (which is first
+    /// normalized to its visible head). `n == 0` yields that head.
+    #[must_use]
+    pub fn nth_visible_from(&self, from: usize, n: usize) -> usize {
+        let mut line = self.visible_head_of(from);
+        for _ in 0..n {
+            line = self.next_visible(line);
+        }
+        line
+    }
+
+    /// The line `n` visible steps back from `from` (first normalized to
+    /// its visible head), saturating at line 0.
+    #[must_use]
+    pub fn nth_visible_back(&self, from: usize, n: usize) -> usize {
+        let mut line = self.visible_head_of(from);
+        for _ in 0..n {
+            if line == 0 {
+                break;
+            }
+            line = self.prev_visible(line);
+        }
+        line
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// A 40-line buffer of `"L<n>\n"`-ish rows, 8 bytes each, so line
+    /// `n` starts at `8n` and its content ends at `8n + 7`.
+    fn line_of(offset: Position) -> usize {
+        (offset / 8) as usize
+    }
+
+    /// The fold that hides lines `first..=last` in that fixture.
+    fn fold(head: usize, last_hidden: usize) -> ByteRange {
+        ByteRange {
+            start: (head as u64) * 8 + 7,
+            end: (last_hidden as u64) * 8 + 7,
+        }
+    }
+
+    fn map(folds: &[ByteRange]) -> VisibleLineMap {
+        VisibleLineMap::build(folds, line_of)
+    }
+
+    #[test]
+    fn empty_map_is_identity() {
+        let m = map(&[]);
+        assert!(m.is_identity());
+        assert!(!m.is_hidden(5));
+        assert_eq!(m.visible_head_of(5), 5);
+        assert_eq!(m.next_visible(5), 6);
+        assert_eq!(m.visible_rows_between(0, 10), 10);
+    }
+
+    #[test]
+    fn single_fold_hides_its_interior_only() {
+        // head 2, hidden 3..=6.
+        let m = map(&[fold(2, 6)]);
+        assert!(!m.is_hidden(2));
+        assert!(m.is_head(2));
+        for line in 3..=6 {
+            assert!(m.is_hidden(line), "line {line} should be hidden");
+            assert_eq!(m.visible_head_of(line), 2);
+        }
+        assert!(!m.is_hidden(7));
+        assert_eq!(m.next_visible(2), 7);
+        assert_eq!(m.prev_visible(7), 2);
+        // 0,1,2,7,8,9 visible in [0,10).
+        assert_eq!(m.visible_rows_between(0, 10), 6);
+        assert_eq!(m.visible_line_count(10), 6);
+    }
+
+    #[test]
+    fn nested_folds_resolve_to_the_outermost_visible_head() {
+        // Outer: head 0, hidden 1..=9. Inner: head 3, hidden 4..=6.
+        let m = map(&[fold(0, 9), fold(3, 6)]);
+        assert_eq!(m.visible_head_of(5), 0, "inner head 3 is itself hidden");
+        assert_eq!(m.visible_head_of(3), 0);
+        assert!(m.is_head(0));
+        assert!(!m.is_head(3), "a hidden head renders no row");
+        assert_eq!(m.next_visible(0), 10);
+    }
+
+    #[test]
+    fn shared_head_folds_merge_to_the_longer_reach() {
+        // Two folds on head 4: one hides 5..=6, the other 5..=9.
+        let m = map(&[fold(4, 6), fold(4, 9)]);
+        assert_eq!(m.visible_head_of(9), 4);
+        assert_eq!(m.next_visible(4), 10);
+        assert_eq!(m.visible_position(9, 9 * 8 + 3), fold(4, 6).start);
+    }
+
+    #[test]
+    fn crossing_folds_project_to_the_first_visible_head() {
+        // Round-3 F2: A hides 1..=3 (head 0); B is headed on line 2 and
+        // hides 3..=5. A point on line 5 is directly inside only B, but
+        // B's head is hidden by A — it must resolve to A's head.
+        let m = map(&[fold(0, 3), fold(2, 5)]);
+        assert!(m.is_hidden(5));
+        assert_eq!(m.visible_head_of(5), 0);
+        assert_eq!(
+            m.visible_position(5, 5 * 8 + 4),
+            fold(0, 3).start,
+            "never B's still-hidden range.start"
+        );
+        assert_eq!(m.next_visible(0), 6);
+        assert!(!m.is_head(2), "B's head is hidden, so it heads nothing");
+    }
+
+    #[test]
+    fn adjacent_folds_merge_because_the_later_head_is_hidden() {
+        // A hides 1..=3 (head 0); B is headed on line 3 (hidden by A)
+        // and hides 4..=5. Lines 1..=5 collapse under head 0.
+        let m = map(&[fold(0, 3), fold(3, 5)]);
+        for line in 1..=5 {
+            assert_eq!(m.visible_head_of(line), 0, "line {line}");
+        }
+        assert_eq!(m.next_visible(0), 6);
+    }
+
+    #[test]
+    fn a_visible_line_between_two_folds_keeps_them_separate() {
+        // A hides 1..=3 (head 0); B hides 5..=6 (head 4). Line 4 stays
+        // visible, so the components do not merge.
+        let m = map(&[fold(0, 3), fold(4, 6)]);
+        assert!(!m.is_hidden(4));
+        assert_eq!(m.visible_head_of(3), 0);
+        assert_eq!(m.visible_head_of(6), 4);
+        assert_eq!(m.next_visible(0), 4);
+        assert_eq!(m.next_visible(4), 7);
+    }
+
+    #[test]
+    fn visible_position_leaves_a_visible_byte_alone() {
+        let m = map(&[fold(2, 6)]);
+        assert_eq!(m.visible_position(7, 7 * 8 + 2), 7 * 8 + 2);
+    }
+
+    #[test]
+    fn clamp_view_top_goes_backward_to_the_head() {
+        let m = map(&[fold(2, 6)]);
+        assert_eq!(m.clamp_view_top(5), 2);
+        assert_eq!(m.clamp_view_top(2), 2);
+        assert_eq!(m.clamp_view_top(7), 7);
+    }
+
+    #[test]
+    fn visible_distance_is_symmetric_and_skips_folds() {
+        let m = map(&[fold(2, 6)]);
+        // Visible order: 0,1,2,7,8 — line 8 is 4 visible steps from 0.
+        assert_eq!(m.visible_distance(0, 8), 4);
+        assert_eq!(m.visible_distance(8, 0), 4);
+        assert_eq!(m.visible_distance(2, 7), 1);
+    }
+
+    #[test]
+    fn nth_visible_walks_forward_and_back_over_folds() {
+        let m = map(&[fold(2, 6)]);
+        assert_eq!(m.nth_visible_from(0, 3), 7);
+        assert_eq!(m.nth_visible_back(8, 4), 0);
+        // A hidden origin normalizes to its head first.
+        assert_eq!(m.nth_visible_from(5, 1), 7);
+        assert_eq!(m.nth_visible_back(5, 1), 1);
+    }
+
+    #[test]
+    fn build_drops_a_fold_that_no_longer_spans_a_line() {
+        // start and end inside one line: nothing to hide.
+        let degenerate = ByteRange { start: 10, end: 12 };
+        assert!(map(&[degenerate]).is_identity());
+    }
+}

src/highlight.rs

@@ -418,7 +418,7 @@ impl View for SyntaxHighlightView {
         "syntax-highlight"
     }
 
-    fn render(&mut self, _buf: &Buffer, viewport: Viewport, cells: &mut CellGrid<'_>) {
+    fn render(&mut self, _buf: &Buffer, viewport: Viewport<'_>, cells: &mut CellGrid<'_>) {
         self.refresh_cache_if_stale();
         let Some(bundle) = self.cache.bundle.clone() else {
             return;
@@ -441,7 +441,11 @@ impl View for SyntaxHighlightView {
         // `compute_highlight_spans_for`) lets narrower captures override.
         for layer in &self.cache.layers {
             for row_offset in 0..max_rows {
-                let line_idx = start_line + row_offset;
+                // Arc 6 Stage 2: row `r` shows the `r`-th VISIBLE line,
+                // matching `TextView::render`'s walk (Q#FD13).
+                let line_idx =
+                    u32::try_from(viewport.line_at_row_offset(start_line as usize, row_offset))
+                        .unwrap_or(u32::MAX);
                 if line_idx >= total_lines {
                     break;
                 }
@@ -598,7 +602,7 @@ impl View for LspStyleView {
         "lsp-style"
     }
 
-    fn render(&mut self, buf: &Buffer, viewport: Viewport, cells: &mut CellGrid<'_>) {
+    fn render(&mut self, buf: &Buffer, viewport: Viewport<'_>, cells: &mut CellGrid<'_>) {
         let Some(path) = buf.file_path() else {
             return; // No path ⇒ no URI ⇒ nothing to look up.
         };
@@ -649,7 +653,10 @@ impl View for LspStyleView {
         let total_lines = line_offsets.len() as u32;
 
         for row_offset in 0..max_rows {
-            let line_idx = start_line + row_offset;
+            // Arc 6 Stage 2: row `r` shows the `r`-th VISIBLE line.
+            let line_idx =
+                u32::try_from(viewport.line_at_row_offset(start_line as usize, row_offset))
+                    .unwrap_or(u32::MAX);
             if line_idx >= total_lines {
                 break;
             }
@@ -1063,6 +1070,7 @@ mod tests {
             cell_origin: CellCoord::new(0, 0),
             cell_size: CellSize::new(1, 20),
             gutter_w: 0,
+            folds: None,
         };
         let registry = state.core.borrow().registry.clone();
         let reg = registry.borrow();
@@ -1160,6 +1168,7 @@ mod tests {
             cell_origin: CellCoord::new(0, 0),
             cell_size: CellSize::new(1, 20),
             gutter_w: 0,
+            folds: None,
         };
         let registry = state.core.borrow().registry.clone();
         let reg = registry.borrow();
@@ -1279,6 +1288,7 @@ mod tests {
             cell_origin: CellCoord::new(0, 0),
             cell_size: CellSize::new(1, 20),
             gutter_w: 0,
+            folds: None,
         };
         let registry = state.core.borrow().registry.clone();
         let reg = registry.borrow();
@@ -1338,6 +1348,7 @@ mod tests {
             cell_origin: CellCoord::new(0, 0),
             cell_size: CellSize::new(rows as u32, cols as u32),
             gutter_w: 0,
+            folds: None,
         };
         let registry = buf; // keep buf alive
         hv.render(&registry, viewport, &mut grid);
@@ -1394,6 +1405,7 @@ mod tests {
             cell_origin: CellCoord::new(0, 0),
             cell_size: CellSize::new(rows as u32, cols as u32),
             gutter_w: 0,
+            folds: None,
         };
         let registry = buf; // keep buf alive
         hv.render(&registry, viewport, &mut grid);
@@ -1458,6 +1470,7 @@ mod tests {
             cell_origin: CellCoord::new(0, 0),
             cell_size: CellSize::new(rows as u32, cols as u32),
             gutter_w: 0,
+            folds: None,
         };
         let registry = buf;
         hv.render(&registry, viewport, &mut grid);
@@ -1510,6 +1523,7 @@ mod tests {
             cell_origin: CellCoord::new(0, 0),
             cell_size: CellSize::new(rows as u32, cols as u32),
             gutter_w: 0,
+            folds: None,
         };
         let registry = buf;
         hv.render(&registry, viewport, &mut grid);
@@ -1572,6 +1586,7 @@ mod tests {
             cell_origin: CellCoord::new(0, 0),
             cell_size: CellSize::new(rows as u32, cols as u32),
             gutter_w: 0,
+            folds: None,
         };
         let registry = buf;
         hv.render(&registry, viewport, &mut grid);

src/hover.rs

@@ -208,7 +208,7 @@ impl HoverView {
 }
 
 impl View for HoverView {
-    fn render(&mut self, _buf: &Buffer, viewport: Viewport, cells: &mut CellGrid<'_>) {
+    fn render(&mut self, _buf: &Buffer, viewport: Viewport<'_>, cells: &mut CellGrid<'_>) {
         let lines: Vec<String> = {
             let guard = self.store.lock().expect("hover store poisoned");
             guard

src/lib.rs

@@ -76,6 +76,7 @@ pub mod editor;
 pub mod editor_core;
 pub mod file_io;
 pub mod fold;
+pub mod fold_view;
 pub mod font_pref;
 pub mod formatting;
 pub mod frontend;

src/lua_bindings/mod.rs

@@ -1308,13 +1308,92 @@ fn run_buffer_edit(
     op: EditOp<'_>,
     bypass_intercept: bool,
 ) -> mlua::Result<crate::rope::Edit> {
+    // Arc 6 Stage 2 (Q#FD19): the interactive-Lua-command unfold seam.
+    // BOTH paths are covered — hooking only `run_managed_edit` would let
+    // an interactive `bypass_intercept` edit escape — but each keys on
+    // *its own* effective edit site (round-5 F1):
+    //
+    // - bypass applies `op` verbatim, so the site is known right here;
+    // - managed runs the intercept chain first, and an intercept may
+    //   legally relocate `pos` / `start` / `end`, so only the op the
+    //   chain settles on names where the edit will land.
     if bypass_intercept {
+        unfold_before_interactive_lua_edit(lua, id, edit_start_of(&op));
         run_bypass_edit(lua, id, op)
     } else {
         run_managed_edit(lua, id, op)
     }
 }
 
+/// Where an [`EditOp`] lands — the `edit.range.start` Q#FD19 keys the
+/// interactive unfold on.
+fn edit_start_of(op: &EditOp<'_>) -> u64 {
+    match op {
+        EditOp::Insert { pos, .. } => *pos,
+        EditOp::Delete { range } | EditOp::Replace { range, .. } => range.start,
+    }
+}
+
+/// Unfold at the **edit site** before an **interactive** Lua-mutator edit
+/// (comment-toggle, yank-pop, and any other `pmacs.buffer.X` mutation a
+/// command body performs on the buffer the user is looking at).
+///
+/// Unfolds only when **all** of:
+///
+/// 1. [`InteractiveCommandOrigin::current`] is `Some(f)` — an
+///    interactive command is running, and `f` is the authenticated
+///    frontend that invoked it. This is the scoped authority that
+///    distinguishes a user command's edit from a plugin's or the data
+///    API's programmatic one; no command-history inference is involved,
+///    and the guard clears even when the Lua command errors.
+/// 2. The edited buffer **is `f`'s active-window buffer**. An explicit
+///    mutation of some *other*, inactive buffer stays programmatic — it
+///    is not an edit the user can see land.
+/// 3. A fold actually contains **`edit_start`** (handled by
+///    [`crate::fold::FoldRegistry::unfold_containing`], a no-op
+///    otherwise).
+///
+/// Condition 3 is the edit site, **not the point** (Q#FD19). A Lua
+/// command is free to edit somewhere other than where the cursor sits:
+/// keying on the cursor would open an unrelated fold when a command
+/// edits elsewhere, and — worse — leave the edit invisible when a
+/// command edits *into* a fold from an outside point. The six
+/// `EditorCore` primitives, yank, and query-replace all edit exactly at
+/// the point, so
+/// [`apply_active_edit`](crate::editor_core::EditorCore::apply_active_edit)'s
+/// funnel keys on the point; only this Lua path can diverge.
+///
+/// "Edit site" means the **effective** one (round-5 F1). On the managed
+/// path a buffer intercept may legally rewrite `pos` / `start` / `end`
+/// before the op is applied, so [`run_managed_edit`] calls this *after*
+/// the intercept chain settles; only [`run_bypass_edit`], which applies
+/// its op verbatim, can name the site up front.
+///
+/// Matches Stage 1's data-API exemption: `pmacs.buffer.insert` called
+/// from a plugin, a hook, or a bare Lua chunk unfolds nothing.
+fn unfold_before_interactive_lua_edit(lua: &Lua, id: BufferId, edit_start: u64) {
+    let Some(origin) = lua
+        .app_data_ref::<InteractiveCommandOrigin>()
+        .and_then(|origin| origin.current())
+    else {
+        return; // programmatic: no interactive command in scope
+    };
+    let Some(folds) = lua.app_data_ref::<crate::fold::SharedFoldRegistry>() else {
+        return;
+    };
+    let Some(core) = lua.app_data_ref::<SharedCore>() else {
+        return;
+    };
+    let core = core.borrow();
+    let Some(window) = core.active_window_for(origin) else {
+        return; // the invoking frontend has no view (nothing to anchor on)
+    };
+    if window.buffer_id != id {
+        return; // an inactive-buffer mutation stays programmatic
+    }
+    folds.unfold_containing(id, edit_start);
+}
+
 fn run_bypass_edit(lua: &Lua, id: BufferId, op: EditOp<'_>) -> mlua::Result<crate::rope::Edit> {
     with_registry_mut(lua, |r| {
         let buf = resolve_mut(r, id)?;
@@ -1366,6 +1445,19 @@ fn run_managed_edit(lua: &Lua, id: BufferId, op: EditOp<'_>) -> mlua::Result<cra
         Ok(current)
     })();
 
+    // Arc 6 Stage 2 (Q#FD19, round-5 F1): the interactive unfold keys on
+    // the EFFECTIVE edit site, so it must run here — after the chain has
+    // settled, before the apply. An intercept may legally relocate `pos`
+    // / `start` / `end`, so widening on the *requested* op would leave a
+    // relocated-into-a-fold edit invisible, and would open an unrelated
+    // fold when the intercept moved the edit out of one. A rejected
+    // chain (`Err`) applies nothing, so it unfolds nothing. The registry
+    // borrow is released at this point; this reads `SharedCore` and the
+    // fold registry only.
+    if let Ok(final_op) = intercept_result.as_ref() {
+        unfold_before_interactive_lua_edit(lua, id, edit_start_of(final_op));
+    }
+
     // Phase 3: re-borrow, restore views, clear mid-edit flag, apply.
     // We restore views and clear the flag even on intercept error,
     // so the buffer is left in a usable state.

src/menu.rs

@@ -378,7 +378,7 @@ impl View for MenuView {
         "context-menu"
     }
 
-    fn render(&mut self, _buf: &Buffer, viewport: Viewport, cells: &mut CellGrid<'_>) {
+    fn render(&mut self, _buf: &Buffer, viewport: Viewport<'_>, cells: &mut CellGrid<'_>) {
         let guard = self.menu.lock().expect("menu mutex poisoned");
         let Some(menu) = guard.as_ref() else {
             return;

src/overlay.rs

@@ -101,7 +101,7 @@ impl StyleSpanOverlay {
 }
 
 impl View for StyleSpanOverlay {
-    fn render(&mut self, _buf: &Buffer, viewport: Viewport, cells: &mut CellGrid<'_>) {
+    fn render(&mut self, _buf: &Buffer, viewport: Viewport<'_>, cells: &mut CellGrid<'_>) {
         for span in &self.spans {
             if span.row >= viewport.cell_size.rows {
                 continue;
@@ -312,7 +312,7 @@ impl View for BufferStyleOverlay {
         Some(Box::new(self.clone()))
     }
 
-    fn render(&mut self, buf: &Buffer, viewport: Viewport, cells: &mut CellGrid<'_>) {
+    fn render(&mut self, buf: &Buffer, viewport: Viewport<'_>, cells: &mut CellGrid<'_>) {
         let spans = self
             .spans
             .lock()
@@ -369,7 +369,7 @@ fn render_buffer_style_span(
     buf: &Buffer,
     line_offsets: &[u64],
     start_line: usize,
-    viewport: Viewport,
+    viewport: Viewport<'_>,
     cells: &mut CellGrid<'_>,
     span: BufferStyleSpan,
 ) {
@@ -379,10 +379,11 @@ fn render_buffer_style_span(
     let first_line = line_at_offset(line_offsets, span.start);
     let last_line = line_at_offset(line_offsets, span.end.saturating_sub(1));
     for line in first_line..=last_line {
-        if line < start_line {
+        // Arc 6 Stage 2: a span on a collapsed line paints nothing (that
+        // line has no row); one below a fold lands on its shifted-up row.
+        let Some(row_offset) = viewport.row_offset_of(start_line, line) else {
             continue;
-        }
+        };
-        let row_offset = (line - start_line) as u32;
         if row_offset >= viewport.cell_size.rows {
             break;
         }
@@ -457,7 +458,7 @@ impl VirtualCellOverlay {
 }
 
 impl View for VirtualCellOverlay {
-    fn render(&mut self, _buf: &Buffer, viewport: Viewport, cells: &mut CellGrid<'_>) {
+    fn render(&mut self, _buf: &Buffer, viewport: Viewport<'_>, cells: &mut CellGrid<'_>) {
         for vc in &self.cells {
             if vc.row >= viewport.cell_size.rows || vc.col >= viewport.cell_size.cols {
                 continue;
@@ -487,13 +488,14 @@ mod tests {
         vec![Cell::default(); (rows * cols) as usize]
     }
 
-    fn viewport(rows: u32, cols: u32) -> Viewport {
+    fn viewport(rows: u32, cols: u32) -> Viewport<'static> {
         Viewport {
             buffer_start: 0,
             buffer_end: u64::MAX,
             cell_origin: CellCoord::new(0, 0),
             cell_size: CellSize::new(rows, cols),
             gutter_w: 0,
+            folds: None,
         }
     }
 

src/overlay_paint.rs

@@ -144,21 +144,47 @@ pub fn paint_other_frontend_overlays(
                 if g >= rect.size.cols { 0 } else { g }
             };
             let text_cols = rect.size.cols.saturating_sub(gutter_w);
+            // Arc 6 Stage 2 (round-2 F2): the fold map belongs to THIS
+            // RECIPIENT WINDOW — built from its own buffer and line
+            // offsets, exactly like its `paint_frame` pass. A split
+            // showing two buffers derives two maps, and a peer in the
+            // unfolded pane is never projected through the folded one's.
+            let folds = crate::fold_view::map_for_window(&state.fold_registry, window);
             // Source's byte position → display coords via THIS
             // recipient window's text_view (the recipient's view
-            // of the buffer).
+            // of the buffer). Q#FD16: a peer cursor on a line this
-            let Some(disp) = window
+            // recipient has collapsed clamps to the head POSITION.
-                .text_view
+            let peer_cursor = match folds.as_ref() {
-                .pos_to_display(buf, presence.snapshot.cursor)
+                Some(map) => map.visible_position(
-            else {
+                    window.text_view.line_at_offset(presence.snapshot.cursor),
+                    presence.snapshot.cursor,
+                ),
+                None => presence.snapshot.cursor,
+            };
+            let Some(disp) = window.text_view.pos_to_display(buf, peer_cursor) else {
                 continue;
             };
             // Filter to viewport visible range. `view_top` is the
             // top visible buffer-line; cells below it are in-frame
-            // until `view_top + inner_rows`.
+            // until `view_top + inner_rows` — counted in VISIBLE rows
-            let row_in_window = match (disp.row as usize).checked_sub(window.view_top) {
+            // once this window has folds.
+            let row_in_window = match folds.as_ref() {
+                Some(map) => {
+                    let top = map.clamp_view_top(window.view_top);
+                    let row = disp.row as usize;
+                    if row < top {
+                        continue;
+                    }
+                    let r = map.visible_rows_between(top, row);
+                    if r >= inner_rows as usize {
+                        continue;
+                    }
+                    r
+                }
+                None => match (disp.row as usize).checked_sub(window.view_top) {
                     Some(r) if r < inner_rows as usize => r,
                     _ => continue,
+                },
             };
             // Column bounds: disp.col is the buffer column; window
             // doesn't horizontally scroll in v1.0, so cells past
@@ -181,7 +207,16 @@ pub fn paint_other_frontend_overlays(
                     (sel.active, sel.anchor)
                 };
                 paint_selection_in_window(
-                    grid, buf, window, rect, inner_rows, gutter_w, lo, hi, color,
+                    grid,
+                    buf,
+                    window,
+                    rect,
+                    inner_rows,
+                    gutter_w,
+                    folds.as_ref(),
+                    lo,
+                    hi,
+                    color,
                 );
             }
         }
@@ -235,10 +270,21 @@ fn paint_selection_in_window(
     rect: Rect,
     inner_rows: u32,
     gutter_w: u32,
+    folds: Option<&crate::fold_view::VisibleLineMap>,
     lo: crate::rope::Position,
     hi: crate::rope::Position,
     color: Color,
 ) {
+    // Arc 6 Stage 2 (Q#FD16): endpoints on a collapsed line project to
+    // their component's head position; hidden interior bytes have no row
+    // and drop below.
+    let (lo, hi) = match folds {
+        Some(map) => (
+            map.visible_position(window.text_view.line_at_offset(lo), lo),
+            map.visible_position(window.text_view.line_at_offset(hi), hi),
+        ),
+        None => (lo, hi),
+    };
     if lo >= hi {
         return;
     }
@@ -261,7 +307,15 @@ fn paint_selection_in_window(
         let Some(disp) = window.text_view.pos_to_display(buf, pos) else {
             break;
         };
-        match (disp.row as usize).checked_sub(window.view_top) {
+        let row_in_window = match folds {
+            Some(map) if map.is_hidden(disp.row as usize) => None,
+            Some(map) => {
+                let top = map.clamp_view_top(window.view_top);
+                (disp.row as usize >= top).then(|| map.visible_rows_between(top, disp.row as usize))
+            }
+            None => (disp.row as usize).checked_sub(window.view_top),
+        };
+        match row_in_window {
             Some(r) if r < inner_rows as usize && disp.col < text_cols => {
                 let grid_row = rect.origin.row + r as u32;
                 let grid_col = rect.origin.col + gutter_w + disp.col;

src/search.rs

@@ -411,7 +411,7 @@ impl View for SearchView {
         "search"
     }
 
-    fn render(&mut self, buf: &Buffer, viewport: Viewport, cells: &mut CellGrid<'_>) {
+    fn render(&mut self, buf: &Buffer, viewport: Viewport<'_>, cells: &mut CellGrid<'_>) {
         let buffer_id = buf.id();
         // Snapshot the matches under the lock, release immediately
         // (same discipline as DiagnosticView).
@@ -439,6 +439,9 @@ impl View for SearchView {
         let line_offsets = crate::diag::compute_line_offsets(&source);
         let start_line_buf =
             crate::diag::line_at_offset(&line_offsets, viewport.buffer_start as u32);
+        // Arc 6 Stage 2: source line → this viewport's row, `None` when
+        // the line is collapsed away (the wash then paints nothing).
+        let row_of = |line: u32| viewport.row_offset_of(start_line_buf as usize, line as usize);
         let max_rows = viewport.cell_size.rows;
         let max_cols = viewport.cell_size.cols;
         let cell_origin = viewport.cell_origin;
@@ -479,17 +482,19 @@ impl View for SearchView {
             // Single-line matches (every literal match) touch one row.
             let first_line = crate::diag::line_at_offset(&line_offsets, m.start as u32);
             // Matches are sorted ascending, so once one starts below the
-            // viewport every later one does too — stop.
+            // viewport every later one does too — stop. Arc 6 Stage 2:
-            if first_line >= start_line_buf.saturating_add(max_rows) {
+            // measured in VISIBLE rows, since a collapse puts a far-away
+            // raw line back on screen; a hidden start yields no offset
+            // and falls through to the per-line skip below.
+            if row_of(first_line).is_some_and(|row| row >= max_rows) {
                 break;
             }
             let last_byte = m.end.saturating_sub(1).max(m.start) as u32;
             let last_line = crate::diag::line_at_offset(&line_offsets, last_byte);
             for line in first_line..=last_line {
-                if line < start_line_buf {
+                let Some(row_offset) = row_of(line) else {
                     continue;
-                }
+                };
-                let row_offset = line - start_line_buf;
                 if row_offset >= max_rows {
                     break;
                 }
@@ -753,6 +758,7 @@ mod tests {
                 cell_origin: CellCoord::new(0, 0),
                 cell_size: CellSize::new(1, 10),
                 gutter_w: 0,
+                folds: None,
             },
             &mut grid,
         );
@@ -781,6 +787,7 @@ mod tests {
                 cell_origin: CellCoord::new(0, 0),
                 cell_size: CellSize::new(1, 10),
                 gutter_w: 0,
+                folds: None,
             },
             &mut grid2,
         );
@@ -822,6 +829,7 @@ mod tests {
                 cell_origin: CellCoord::new(0, 0),
                 cell_size: CellSize::new(rows, cols),
                 gutter_w: 0,
+                folds: None,
             },
             &mut grid,
         );

src/signature.rs

@@ -318,7 +318,7 @@ impl SignatureView {
 }
 
 impl View for SignatureView {
-    fn render(&mut self, _buf: &Buffer, viewport: Viewport, cells: &mut CellGrid<'_>) {
+    fn render(&mut self, _buf: &Buffer, viewport: Viewport<'_>, cells: &mut CellGrid<'_>) {
         let snap = self.snapshot();
         let max_rows = viewport.cell_size.rows;
         let max_cols = viewport.cell_size.cols;

src/text_view.rs

@@ -33,6 +33,10 @@ use crate::view::{DisplayCoord, View, Viewport};
 /// [`TextView::pos_to_display`]; longer prefixes fall back to a heap buffer.
 const STACK_CAP: usize = 256;
 
+/// Trailing marker painted after a collapsed region's head line (Arc 6
+/// Stage 2, Q#FD13). One column wide, so it never disturbs layout.
+pub const FOLD_ELLIPSIS: char = '…';
+
 // ---------------------------------------------------------------------------
 // TextView
 // ---------------------------------------------------------------------------
@@ -204,14 +208,26 @@ impl View for TextView {
         Some(line_start + walked_bytes as u64)
     }
 
-    fn render(&mut self, buf: &Buffer, viewport: Viewport, cells: &mut CellGrid<'_>) {
+    fn render(&mut self, buf: &Buffer, viewport: Viewport<'_>, cells: &mut CellGrid<'_>) {
-        let start_line = self.line_at_offset(viewport.buffer_start);
+        // Arc 6 Stage 2 (Q#FD13): row `r` shows the `r`-th VISIBLE source
+        // line at or after `view_top`; a collapsed region's lines are
+        // skipped entirely and the rows below shift up. Without a fold
+        // map this walk is the pre-folding `start_line + row_offset`
+        // identity. Folding is deliberately not an overlay — overlays
+        // repaint cells, they cannot delete rows.
+        let folds = viewport.folds.filter(|m| !m.is_identity());
+        // `view_top` is clamped backward before the frame, but a caller
+        // that hands us a hidden start still gets its head.
+        let start_line = {
+            let raw = self.line_at_offset(viewport.buffer_start);
+            folds.map_or(raw, |m| m.visible_head_of(raw))
+        };
         let max_rows = viewport.cell_size.rows;
         let max_cols = viewport.cell_size.cols;
         let origin = viewport.cell_origin;
 
+        let mut line = start_line;
         for row_offset in 0..max_rows {
-            let line = start_line + row_offset as usize;
             let cell_row = origin.row + row_offset;
 
             // Always clear the visible row first so previous content does
@@ -222,8 +238,10 @@ impl View for TextView {
             if line >= self.line_count() {
                 continue;
             }
+            let this_line = line;
+            line = folds.map_or(this_line + 1, |m| m.next_visible(this_line));
 
-            let line_bytes = self.read_line_bytes(buf, line);
+            let line_bytes = self.read_line_bytes(buf, this_line);
             let Ok(s) = std::str::from_utf8(&line_bytes) else {
                 continue;
             };
@@ -267,6 +285,23 @@ impl View for TextView {
                 }
                 col += width;
             }
+
+            // The head of a collapsed region carries a trailing ellipsis
+            // in the CONTENT area (Q#FD13/FD20): the authoritative,
+            // layout-neutral fold indicator, present in every gutter
+            // state, clipped like any long line.
+            if folds.is_some_and(|m| m.is_head(this_line)) {
+                for marker in [' ', FOLD_ELLIPSIS] {
+                    if col >= max_cols {
+                        break;
+                    }
+                    let cell = cells.at(CellCoord::new(cell_row, origin.col + col));
+                    cell.glyph = Glyph::Char(marker);
+                    cell.style = Style::default();
+                    cell.attachment = None;
+                    col += 1;
+                }
+            }
         }
     }
 }
@@ -534,6 +569,7 @@ mod tests {
                 cell_origin: CellCoord::new(0, 0),
                 cell_size: CellSize::new(1, 16),
                 gutter_w: 0,
+                folds: None,
             },
             &mut grid,
         );
@@ -562,6 +598,7 @@ mod tests {
                 cell_origin: CellCoord::new(0, 0),
                 cell_size: CellSize::new(1, 16),
                 gutter_w: 0,
+                folds: None,
             },
             &mut grid,
         );
@@ -593,6 +630,7 @@ mod tests {
                 cell_origin: CellCoord::new(0, 0),
                 cell_size: CellSize::new(5, 5),
                 gutter_w: 0,
+                folds: None,
             },
             &mut grid,
         );
@@ -625,6 +663,7 @@ mod tests {
                 cell_origin: CellCoord::new(0, 0),
                 cell_size: CellSize::new(1, 5),
                 gutter_w: 0,
+                folds: None,
             },
             &mut grid,
         );

src/view.rs

@@ -40,6 +40,7 @@
 
 use crate::buffer::{Buffer, BufferError, BufferId, EditOp};
 use crate::cell::{CellCoord, CellGrid, CellSize};
+use crate::fold_view::VisibleLineMap;
 use crate::rope::{Edit, Position};
 
 // ---------------------------------------------------------------------------
@@ -127,7 +128,7 @@ impl DisplayCoord {
 /// cell origin) and hands it to the view. The view fills cells inside that
 /// origin/size window.
 #[derive(Copy, Clone, Eq, PartialEq, Debug)]
-pub struct Viewport {
+pub struct Viewport<'a> {
     /// First byte in the buffer to consider rendering.
     pub buffer_start: Position,
     /// One past the last byte to consider.
@@ -142,6 +143,58 @@ pub struct Viewport {
     /// `cell_origin.col - gutter_w`; overlays that only touch the text area
     /// ignore it (the origin is already shifted past the gutter).
     pub gutter_w: u32,
+    /// The rendering window's collapsed regions (Arc 6 Stage 2, Q#FD12),
+    /// or `None` when this window's buffer has no folds — the unfolded
+    /// path then stays byte-identical to the pre-folding renderer.
+    ///
+    /// Borrowed rather than owned so `Viewport` stays [`Copy`]: the frame
+    /// builds **one map per rendered window** (never one per frame — a
+    /// split may show different buffers) and hands the same shared
+    /// reference to every painter of that window.
+    pub folds: Option<&'a VisibleLineMap>,
+}
+
+impl Viewport<'_> {
+    /// Row offset within this viewport for source `line`, given the
+    /// viewport's first (visible) source line.
+    ///
+    /// `None` when `line` is above `start_line` or collapsed away — a
+    /// hidden line simply has no row, so its painter skips it. Without a
+    /// map this is the identity `line - start_line` every consumer used
+    /// before folding.
+    ///
+    /// The result is monotonically non-decreasing in `line`, so a caller
+    /// that bounds its walk with `row_offset >= rows` may still `break`.
+    #[must_use]
+    pub fn row_offset_of(self, start_line: usize, line: usize) -> Option<u32> {
+        let raw = line.checked_sub(start_line)?;
+        let rows = match self.folds {
+            Some(map) if !map.is_identity() => {
+                if map.is_hidden(line) {
+                    return None;
+                }
+                map.visible_rows_between(start_line, line)
+            }
+            _ => raw,
+        };
+        u32::try_from(rows).ok()
+    }
+
+    /// The inverse of [`Self::row_offset_of`]: the source line rendered
+    /// at `row_offset`, given the viewport's first (visible) source line.
+    ///
+    /// Painters that walk rows rather than spans (the syntax and LSP
+    /// style views) use this; the result may exceed the buffer's line
+    /// count, which those callers already bound.
+    #[must_use]
+    pub fn line_at_row_offset(self, start_line: usize, row_offset: u32) -> usize {
+        match self.folds {
+            Some(map) if !map.is_identity() => {
+                map.nth_visible_from(start_line, row_offset as usize)
+            }
+            _ => start_line + row_offset as usize,
+        }
+    }
 }
 
 // ---------------------------------------------------------------------------
@@ -209,7 +262,7 @@ pub trait View {
     ///
     /// Default: no-op (used by views that participate in `on_edit` but not
     /// in rendering).
-    fn render(&mut self, _buf: &Buffer, _viewport: Viewport, _cells: &mut CellGrid<'_>) {}
+    fn render(&mut self, _buf: &Buffer, _viewport: Viewport<'_>, _cells: &mut CellGrid<'_>) {}
 
     /// Translate a buffer byte position to a display coordinate, if the
     /// view holds a meaningful mapping for that position.
@@ -277,8 +330,45 @@ mod tests {
     #[test]
     fn viewport_is_copy() {
         // Compile-time assertion: Viewport must be Copy so the frontend can
-        // hand the same descriptor to multiple views without ceremony.
+        // hand the same descriptor to multiple views without ceremony. Arc 6
+        // Stage 2 (Bet B7) keeps this true by borrowing the fold map — a
+        // shared reference is itself `Copy`.
         fn assert_copy<T: Copy>() {}
-        assert_copy::<Viewport>();
+        assert_copy::<Viewport<'_>>();
+    }
+
+    #[test]
+    fn row_offset_without_folds_is_the_identity_map() {
+        let vp = Viewport {
+            buffer_start: 0,
+            buffer_end: 0,
+            cell_origin: CellCoord::new(0, 0),
+            cell_size: CellSize::new(10, 10),
+            gutter_w: 0,
+            folds: None,
+        };
+        assert_eq!(vp.row_offset_of(4, 4), Some(0));
+        assert_eq!(vp.row_offset_of(4, 9), Some(5));
+        assert_eq!(vp.row_offset_of(4, 3), None, "above the viewport");
+    }
+
+    #[test]
+    fn row_offset_skips_hidden_lines_and_compacts_rows() {
+        // Fold heading line 1, hiding lines 2..=4 (8-byte lines).
+        let map =
+            VisibleLineMap::build(&[pmacs_protocol::ByteRange { start: 15, end: 39 }], |off| {
+                (off / 8) as usize
+            });
+        let vp = Viewport {
+            buffer_start: 0,
+            buffer_end: 0,
+            cell_origin: CellCoord::new(0, 0),
+            cell_size: CellSize::new(10, 10),
+            gutter_w: 0,
+            folds: Some(&map),
+        };
+        assert_eq!(vp.row_offset_of(0, 1), Some(1), "the head keeps its row");
+        assert_eq!(vp.row_offset_of(0, 3), None, "hidden lines have no row");
+        assert_eq!(vp.row_offset_of(0, 5), Some(2), "rows below shift up");
     }
 }

src/window.rs

@@ -324,6 +324,28 @@ pub struct FrontendView {
     /// `layout` references (invariant: `layout.iter_ids()` contains
     /// `active`).
     pub active: WindowId,
+    /// Whether this frontend's display *projects* folds — i.e. whether
+    /// it collapses hidden lines away (Arc 6 Stage 2, Q#FD21).
+    ///
+    /// Motion, paging, wheel scrolling, click inverses, and the
+    /// auto-scroll clamp all live in shared
+    /// [`EditorCore`](crate::editor_core::EditorCore) code, but Stage 2
+    /// collapses only the **grid** renderer — a `semantic_render` (GPU)
+    /// session still displays every source line until Stage 3, and both
+    /// kinds may attach to one buffer at once over the same shared fold
+    /// store. Reckoning in visible lines unconditionally would make that
+    /// GPU session's cursor skip lines it is still showing, so every
+    /// command/event-time visible-line reckoning is gated on the
+    /// **acting** frontend's flag. Render-time clamps need no gate: a
+    /// semantic session never enters `paint_frame`.
+    ///
+    /// Set at attach from the negotiated selected-render bit (grid ⇒
+    /// `true`, semantic ⇒ `false`), cleared with the view at detach, and
+    /// `true` for [`FrontendId::LOCAL`](crate::protocol::FrontendId).
+    /// Deliberately has no `Default`: every construction site chooses
+    /// explicitly, so the projection is never inferred from a
+    /// `FrontendId` (**Bet B8**).
+    pub fold_projection: bool,
 }
 
 impl Layout {

tests/compile_mode_acceptance.rs

@@ -250,6 +250,7 @@ fn render_active_window_to_grid(
         cell_origin: rect.origin,
         cell_size: CellSize::new(rect.size.rows, rect.size.cols),
         gutter_w: 0,
+        folds: None,
     };
     let mut grid = CellGrid {
         cells: &mut backing,

tests/m4_acceptance.rs

@@ -498,6 +498,7 @@ fn render_active_window_to_grid(
         cell_origin: rect.origin,
         cell_size: CellSize::new(rect.size.rows, rect.size.cols),
         gutter_w: 0,
+        folds: None,
     };
     let mut grid = CellGrid {
         cells: &mut backing,

tests/statusline_segments_acceptance.rs

@@ -464,6 +464,7 @@ fn a05_08_evaluator_latches_reentrancy_contexts_and_mutation_guards() {
             pmacs::window::FrontendView {
                 layout: pmacs::window::Layout::single(window_id),
                 active: window_id,
+                fold_projection: true,
             },
         );
     }

tests/tab_width_acceptance.rs

@@ -8,13 +8,14 @@ use pmacs::overlay::{BufferStyleOverlay, BufferStyleSpan, SharedBufferStyleSpans
 use pmacs::text_view::TextView;
 use pmacs::view::{DisplayCoord, View, Viewport};
 
-fn viewport(rows: u32, cols: u32, buffer_end: u64) -> Viewport {
+fn viewport(rows: u32, cols: u32, buffer_end: u64) -> Viewport<'static> {
     Viewport {
         buffer_start: 0,
         buffer_end,
         cell_origin: CellCoord::new(0, 0),
         cell_size: CellSize::new(rows, cols),
         gutter_w: 0,
+        folds: None,
     }
 }
 

tests/vterm_stage3_acceptance.rs

@@ -81,6 +81,7 @@ fn attach_view(
         FrontendView {
             layout: Layout::single(window_id),
             active: window_id,
+            fold_projection: true,
         },
     );
     window_id