diff --git a/src/completion.rs b/src/completion.rs
index 70bc4ab..950b354 100644
--- a/src/completion.rs
+++ b/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 {
-        return None; // anchor scrolled above the viewport
-    }
-    let anchor_row = anchor_line - start_line;
+    // Arc 6 Stage 2: the popup anchors on the anchor byte's VISIBLE row,
+    // so a completion below a collapsed region lands on the right row;
+    // an anchor inside a collapse has no row and paints nothing.
+    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");
diff --git a/src/daemon.rs b/src/daemon.rs
index fe28112..8a3c727 100644
--- a/src/daemon.rs
+++ b/src/daemon.rs
@@ -1545,7 +1545,16 @@ fn handle_session_established(
     // 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).
-    let scratch_view = build_fresh_frontend_view(editor);
+    //
+    // 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 scratch_view = build_fresh_frontend_view(
+        editor,
+        !session_state.negotiated_capabilities.semantic_render,
+    );
     editor
         .core
         .borrow_mut()
@@ -2622,7 +2631,13 @@ 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();
@@ -2651,6 +2666,7 @@ fn build_fresh_frontend_view(editor: &mut EditorState) -> crate::window::Fronten
     FrontendView {
         layout: Layout::single(id),
         active: id,
+        fold_projection,
     }
 }
 
@@ -3442,6 +3458,7 @@ mod tests {
                 FrontendView {
                     layout: Layout::single(wid),
                     active: wid,
+                    fold_projection: true,
                 },
             );
         }
diff --git a/src/desktop.rs b/src/desktop.rs
index 23ff5ac..7f44524 100644
--- a/src/desktop.rs
+++ b/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
diff --git a/src/diag.rs b/src/diag.rs
index cd2909e..88aa6cc 100644
--- a/src/diag.rs
+++ b/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 row_offset >= max_rows {
+                };
+                if sign_row >= 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,
         );
diff --git a/src/editor.rs b/src/editor.rs
index 00f0fd4..3cf59e6 100644
--- a/src/editor.rs
+++ b/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);
-        let target = crate::view::DisplayCoord::new(display_row as u32, local_col);
+        // Arc 6 Stage 2 (Q#FD16/FD21): grid row `k` in this window shows
+        // 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 {
-            old_top.saturating_sub(magnitude)
-        } else {
-            old_top.saturating_add(magnitude).min(max_top)
+        let new_top = match folds.as_ref() {
+            Some(map) if scroll_up => map.nth_visible_back(old_top, magnitude),
+            Some(map) => map
+                .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.
-        let view_shift = if scroll_up {
-            old_top.saturating_sub(new_top)
-        } else {
-            new_top.saturating_sub(old_top)
+        // many lines the view actually shifted (counted in VISIBLE
+        // lines once this window folds).
+        let view_shift = match folds.as_ref() {
+            Some(map) => map.visible_distance(old_top, new_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 {
-                    cur_row.saturating_sub(view_shift)
-                } else {
-                    cur_row.saturating_add(view_shift).min(max_top)
+                let target_row_usize = match folds.as_ref() {
+                    Some(map) if scroll_up => map.nth_visible_back(cur_row, view_shift),
+                    Some(map) => map
+                        .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",
             );
@@ -2867,10 +2905,31 @@ pub fn paint_frame(
                 .text_view
                 .pos_to_display(buf, aw.cursor)
                 .map_or(0, |d| d.row as usize);
-            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 {
-                aw.view_top = cursor_row + 1 - inner_rows 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 {
+                        aw.view_top = cursor_row + 1 - inner_rows as usize;
+                    }
+                }
             }
         }
     }
@@ -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(
-            window.view_top,
-            inner_rows as usize,
-            window.text_view.line_count(),
-            coord.row as usize,
-        );
+        // 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,
+                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)?;
-    if (disp.row as usize) < aw.view_top || (disp.row as usize) >= aw.view_top + inner_rows as usize
-    {
+    // Arc 6 Stage 2 (Q#FD16, round-2 F3): a logical cursor on a hidden
+    // 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;
-    let last_row = first_row.saturating_add(inner_rows as usize);
-    for display_row in first_row..last_row {
+    // Row `r` shows the `r`-th VISIBLE line at or after `view_top`.
+    let mut next_line = folds.map_or(window.view_top, |map| map.visible_head_of(window.view_top));
+    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.
diff --git a/src/editor_core.rs b/src/editor_core.rs
index 5afbe90..2a15e32 100644
--- a/src/editor_core.rs
+++ b/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.
@@ -1264,6 +1311,16 @@ 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.
@@ -1541,27 +1598,42 @@ 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();
         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
-                .active_window()
+            let aw = self.active_window();
+            let coord = aw
                 .text_view
                 .pos_to_display(buffer, cursor)
                 .unwrap_or_default();
-            if coord.row == 0 {
+            let from_row = folds.as_ref().map_or(coord.row as usize, |map| {
+                map.visible_head_of(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 new_pos = self
-                .active_window()
-                .text_view
-                .display_to_pos(buffer, target);
+            let Ok(target_row) = u32::try_from(target_row) else {
+                return;
+            };
+            let target = DisplayCoord::new(target_row, goal);
+            let new_pos = aw.text_view.display_to_pos(buffer, target);
             (goal, new_pos)
         };
         let (goal, new_pos) = result;
@@ -1573,28 +1645,35 @@ 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();
         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
-                .active_window()
+            let aw = self.active_window();
+            let coord = aw
                 .text_view
                 .pos_to_display(buffer, cursor)
                 .unwrap_or_default();
-            let next_row = coord.row + 1;
-            if (next_row as usize) >= self.active_window().text_view.line_count() {
+            let from_row = folds.as_ref().map_or(coord.row as usize, |map| {
+                map.visible_head_of(coord.row as usize)
+            });
+            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
-                .active_window()
-                .text_view
-                .display_to_pos(buffer, target);
+            let new_pos = aw.text_view.display_to_pos(buffer, target);
             (goal, new_pos)
         };
         let (goal, new_pos) = result;
@@ -1743,6 +1822,9 @@ 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.
+        let folds = self.fold_map_active();
         let step = self.page_step();
         let cursor = self.active_window().cursor;
         let view_top = self.active_window().view_top;
@@ -1755,12 +1837,25 @@ 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();
@@ -1771,12 +1866,16 @@ 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();
         let step = self.page_step();
         let cursor = self.active_window().cursor;
         let view_top = self.active_window().view_top;
@@ -1790,10 +1889,22 @@ 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();
@@ -1835,15 +1946,21 @@ impl EditorCore {
         aw.goal_col = None;
     }
 
-    /// Dispatch-layer pre-edit unfold (Arc 6, Q#FD5). Before a
-    /// command-path point-anchored edit (the six primitives below),
-    /// unfold every fold containing the active point so a self-insert or
-    /// delete inside a collapsed region reveals it rather than landing
-    /// invisibly. Keyed on the authenticated source frontend's active
-    /// point (this is `active_window().cursor`), not the transport. A
-    /// no-op when the buffer has no folds. Interactive Lua-command edits
-    /// (yank/query-replace/comment) reach the buffer through a different
-    /// path and are a named Stage 2 widening; CRDT-origin is Stage 3.
+    /// Pre-edit unfold (Arc 6, Q#FD5 / Stage 2 Q#FD19). Before a local
+    /// point-anchored edit, unfold every fold containing the active point
+    /// so an edit inside a collapsed region reveals it rather than
+    /// landing invisibly. Keyed on the authenticated source frontend's
+    /// active point (`active_window().cursor`), not the transport. A
+    /// no-op when the buffer has no folds.
+    ///
+    /// **Stage 2 widening:** Stage 1 called this from each of the six
+    /// `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).
     fn unfold_before_point_edit(&self) {
         let id = self.active_buffer_id();
         let point = self.active_window().cursor;
@@ -1855,7 +1972,6 @@ 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);
@@ -1891,7 +2007,6 @@ 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
@@ -1933,7 +2048,6 @@ 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 {
@@ -1955,7 +2069,6 @@ 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();
@@ -1979,7 +2092,6 @@ 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 {
@@ -2007,7 +2119,6 @@ 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();
@@ -3345,6 +3456,7 @@ mod tests {
             FrontendView {
                 layout: Layout::single(win_id),
                 active: win_id,
+                fold_projection: true,
             },
         );
         win_id
diff --git a/src/fold_view.rs b/src/fold_view.rs
new file mode 100644
index 0000000..bebaff6
--- /dev/null
+++ b/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());
+    }
+}
diff --git a/src/highlight.rs b/src/highlight.rs
index 8a8914d..e7e5a98 100644
--- a/src/highlight.rs
+++ b/src/highlight.rs
@@ -413,7 +413,7 @@ 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;
@@ -436,7 +436,11 @@ 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;
                 }
@@ -593,7 +597,7 @@ 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.
         };
@@ -644,7 +648,10 @@ 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;
             }
@@ -1058,6 +1065,7 @@ 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();
@@ -1155,6 +1163,7 @@ 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();
@@ -1274,6 +1283,7 @@ 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();
@@ -1333,6 +1343,7 @@ 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);
@@ -1389,6 +1400,7 @@ 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);
diff --git a/src/hover.rs b/src/hover.rs
index 6de7660..d32a86b 100644
--- a/src/hover.rs
+++ b/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
diff --git a/src/lib.rs b/src/lib.rs
index 392f7b7..e3971a3 100644
--- a/src/lib.rs
+++ b/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;
diff --git a/src/lua_bindings/mod.rs b/src/lua_bindings/mod.rs
index 7e84899..4747a7c 100644
--- a/src/lua_bindings/mod.rs
+++ b/src/lua_bindings/mod.rs
@@ -1308,6 +1308,11 @@ 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.
+    // Hooked HERE — above both `run_managed_edit` and `run_bypass_edit` —
+    // so an interactive command that passes `bypass_intercept` does not
+    // escape the widening.
+    unfold_before_interactive_lua_edit(lua, id);
     if bypass_intercept {
         run_bypass_edit(lua, id, op)
     } else {
@@ -1315,6 +1320,50 @@ fn run_buffer_edit(
     }
 }
 
+/// Unfold at the point 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 at the user's point.
+/// 3. A fold actually contains that point (handled by
+///    [`crate::fold::FoldRegistry::unfold_containing`], which is a no-op
+///    otherwise).
+///
+/// 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) {
+    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, window.cursor);
+}
+
 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)?;
diff --git a/src/menu.rs b/src/menu.rs
index 5ce07a5..7de7bf7 100644
--- a/src/menu.rs
+++ b/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;
diff --git a/src/overlay.rs b/src/overlay.rs
index 5d41e28..cdbce35 100644
--- a/src/overlay.rs
+++ b/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,
         }
     }
 
diff --git a/src/overlay_paint.rs b/src/overlay_paint.rs
index af0ee7f..b16daae 100644
--- a/src/overlay_paint.rs
+++ b/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).
-            let Some(disp) = window
-                .text_view
-                .pos_to_display(buf, presence.snapshot.cursor)
-            else {
+            // of the buffer). Q#FD16: a peer cursor on a line this
+            // recipient has collapsed clamps to the head POSITION.
+            let peer_cursor = match folds.as_ref() {
+                Some(map) => map.visible_position(
+                    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`.
-            let row_in_window = match (disp.row as usize).checked_sub(window.view_top) {
-                Some(r) if r < inner_rows as usize => r,
-                _ => continue,
+            // until `view_top + inner_rows` — counted in VISIBLE rows
+            // 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;
diff --git a/src/search.rs b/src/search.rs
index 0ff1211..b77d231 100644
--- a/src/search.rs
+++ b/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.
-            if first_line >= start_line_buf.saturating_add(max_rows) {
+            // viewport every later one does too — stop. Arc 6 Stage 2:
+            // 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,
         );
diff --git a/src/signature.rs b/src/signature.rs
index 495e8bf..6121895 100644
--- a/src/signature.rs
+++ b/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;
diff --git a/src/text_view.rs b/src/text_view.rs
index b94e03d..aae8c06 100644
--- a/src/text_view.rs
+++ b/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<'_>) {
-        let start_line = self.line_at_offset(viewport.buffer_start);
+    fn render(&mut self, buf: &Buffer, viewport: Viewport<'_>, cells: &mut CellGrid<'_>) {
+        // 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,
         );
diff --git a/src/view.rs b/src/view.rs
index 313f411..424fd9e 100644
--- a/src/view.rs
+++ b/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");
     }
 }
diff --git a/src/window.rs b/src/window.rs
index 22add47..e1162c0 100644
--- a/src/window.rs
+++ b/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 {
diff --git a/tests/compile_mode_acceptance.rs b/tests/compile_mode_acceptance.rs
index 56be46c..377b094 100644
--- a/tests/compile_mode_acceptance.rs
+++ b/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,
diff --git a/tests/folding_stage2_acceptance.rs b/tests/folding_stage2_acceptance.rs
new file mode 100644
index 0000000..4f1a657
--- /dev/null
+++ b/tests/folding_stage2_acceptance.rs
@@ -0,0 +1,1265 @@
+// folding_stage2_acceptance.rs --- Arc 6 Stage 2 acceptance
+// (docs/folding-stage2-framing.md, acceptance items 1–14).
+
+//! Grid (daemon-rendered) collapse.
+//!
+//! Every claim about what the user sees is asserted on the **rendered
+//! cell grid** through the real `paint_frame` — the same pipeline the
+//! daemon ships to a terminal client — not on the fold store or on a
+//! painter in isolation. Folds are created through the real
+//! `pmacs.fold` data API so the stored ranges are normalized exactly as
+//! a user command would leave them.
+//!
+//! The fixture buffer is twelve four-byte lines (`L00\n` … `L11\n`), so
+//! line `n` starts at `4n` and its content ends at `4n + 3` — which is
+//! precisely a fold's `ByteRange::start` for a head line `n`.
+
+use crossterm::event::{
+    KeyCode, KeyEvent, KeyEventKind, KeyEventState, KeyModifiers, MouseEvent, MouseEventKind,
+};
+use pmacs::buffer::{BufferId, EditOp};
+use pmacs::cell::{Cell, CellCoord, CellGrid, CellSize, Glyph};
+use pmacs::editor::EditorState;
+use pmacs::protocol::FrontendId;
+use pmacs::window::{FrontendView, Layout, Window, WindowId};
+
+// ---------------------------------------------------------------------------
+// Harness
+// ---------------------------------------------------------------------------
+
+/// Terminal geometry: 12 rows × 40 cols. `paint_frame` reserves the last
+/// row for the status line and each window's last row for its mode line,
+/// so a single window paints text into grid rows 0..=9.
+const ROWS: u32 = 12;
+const COLS: u32 = 40;
+const TEXT_ROWS: u32 = 10;
+
+/// Bytes per fixture line (`"Lnn\n"`).
+const LINE_BYTES: u64 = 4;
+
+fn fixture() -> String {
+    (0..12).fold(String::new(), |mut acc, n| {
+        use std::fmt::Write as _;
+        let _ = writeln!(acc, "L{n:02}");
+        acc
+    })
+}
+
+/// A taller fixture for paging/scrolling: 80 five-byte lines
+/// (`"Mnnn\n"`), so line `n` starts at `5n`.
+fn long_fixture() -> String {
+    (0..80).fold(String::new(), |mut acc, n| {
+        use std::fmt::Write as _;
+        let _ = writeln!(acc, "M{n:03}");
+        acc
+    })
+}
+
+/// Content-end byte of fixture line `n` — a fold's head `start`.
+fn end_of(line: usize) -> u64 {
+    line as u64 * LINE_BYTES + 3
+}
+
+fn editor() -> EditorState {
+    let s = EditorState::new();
+    exec(&s, "pmacs.lsp.config = {}");
+    s
+}
+
+fn exec(s: &EditorState, src: &str) {
+    s.lua_host.lua().load(src.to_string()).exec().unwrap();
+}
+
+fn eval<T: mlua::FromLuaMulti>(s: &EditorState, src: &str) -> T {
+    s.lua_host.lua().load(src.to_string()).eval().unwrap()
+}
+
+fn active_id(s: &EditorState) -> BufferId {
+    s.core.borrow().active_buffer_id()
+}
+
+/// Insert `text` at the start of `id` and notify every window, so the
+/// per-window `TextView` line index matches what we are about to paint.
+fn seed(s: &EditorState, id: BufferId, text: &str) {
+    let edit = {
+        let core = s.core.borrow();
+        let registry = core.registry.clone();
+        let mut reg = registry.borrow_mut();
+        reg.get_mut(id)
+            .unwrap()
+            .apply_edit(EditOp::Insert {
+                pos: 0,
+                bytes: text.as_bytes(),
+            })
+            .unwrap()
+    };
+    s.core.borrow_mut().notify_buffer_edit(id, &edit);
+}
+
+/// A buffer holding the fixture, seeded into the active window.
+fn seeded() -> (EditorState, BufferId) {
+    let s = editor();
+    let id = active_id(&s);
+    seed(&s, id, &fixture());
+    (s, id)
+}
+
+/// Collapse fixture lines `head + 1 ..= last_hidden` through the real
+/// `pmacs.fold` data API. Panics if the range is rejected.
+fn fold_lines(s: &EditorState, buffer: &str, head: usize, last_hidden: usize) {
+    let ok: bool = eval(
+        s,
+        &format!(
+            "return pmacs.fold.fold({buffer}, {{ start = {}, ['end'] = {} }})",
+            end_of(head),
+            end_of(last_hidden)
+        ),
+    );
+    assert!(ok, "fold({head}..={last_hidden}) must be accepted");
+}
+
+/// Collapse in the *active* buffer.
+fn fold_active(s: &EditorState, head: usize, last_hidden: usize) {
+    fold_lines(s, "pmacs.window.buffer()", head, last_hidden);
+}
+
+/// Paint the full frame for `fid` and return the backing cells.
+fn paint_for(state: &EditorState, fid: FrontendId) -> Vec<Cell> {
+    let mut backing = vec![Cell::default(); (ROWS * COLS) as usize];
+    let mut grid = CellGrid {
+        cells: &mut backing,
+        stride: COLS,
+        size: CellSize::new(ROWS, COLS),
+    };
+    let _cursor = pmacs::editor::paint_frame(
+        state,
+        fid,
+        &std::collections::HashMap::new(),
+        &mut grid,
+        CellSize::new(ROWS, COLS),
+    );
+    backing
+}
+
+fn paint(state: &EditorState) -> Vec<Cell> {
+    paint_for(state, FrontendId::LOCAL)
+}
+
+/// Paint the full frame and return the terminal caret cell.
+fn paint_caret(state: &EditorState) -> Option<CellCoord> {
+    let mut backing = vec![Cell::default(); (ROWS * COLS) as usize];
+    let mut grid = CellGrid {
+        cells: &mut backing,
+        stride: COLS,
+        size: CellSize::new(ROWS, COLS),
+    };
+    pmacs::editor::paint_frame(
+        state,
+        FrontendId::LOCAL,
+        &std::collections::HashMap::new(),
+        &mut grid,
+        CellSize::new(ROWS, COLS),
+    )
+}
+
+fn at(cells: &[Cell], row: u32, col: u32) -> &Cell {
+    &cells[(row * COLS + col) as usize]
+}
+
+fn row_text(cells: &[Cell], row: u32) -> String {
+    (0..COLS)
+        .map(|c| match at(cells, row, c).glyph {
+            Glyph::Char(ch) => ch,
+            _ => ' ',
+        })
+        .collect::<String>()
+        .trim_end()
+        .to_string()
+}
+
+/// The text rows of a single-window frame, trimmed.
+fn text_rows(cells: &[Cell]) -> Vec<String> {
+    (0..TEXT_ROWS).map(|r| row_text(cells, r)).collect()
+}
+
+fn key(code: KeyCode, mods: KeyModifiers) -> KeyEvent {
+    KeyEvent {
+        code,
+        modifiers: mods,
+        kind: KeyEventKind::Press,
+        state: KeyEventState::NONE,
+    }
+}
+
+fn press(s: &mut EditorState, code: KeyCode) {
+    s.dispatch_key(FrontendId::LOCAL, key(code, KeyModifiers::NONE));
+}
+
+fn ctrl(s: &mut EditorState, c: char) {
+    s.dispatch_key(
+        FrontendId::LOCAL,
+        key(KeyCode::Char(c), KeyModifiers::CONTROL),
+    );
+}
+
+fn alt(s: &mut EditorState, c: char) {
+    s.dispatch_key(FrontendId::LOCAL, key(KeyCode::Char(c), KeyModifiers::ALT));
+}
+
+fn type_str(s: &mut EditorState, text: &str) {
+    for ch in text.chars() {
+        s.dispatch_key(
+            FrontendId::LOCAL,
+            key(KeyCode::Char(ch), KeyModifiers::NONE),
+        );
+    }
+}
+
+fn cursor_line(s: &EditorState) -> usize {
+    s.core.borrow().cursor_line()
+}
+
+fn set_cursor(s: &EditorState, pos: u64) {
+    s.core.borrow_mut().set_cursor_byte(pos);
+}
+
+fn view_top(s: &EditorState) -> usize {
+    s.core.borrow().active_window().view_top
+}
+
+fn fold_count(s: &EditorState) -> usize {
+    let id = active_id(s);
+    s.fold_registry.folds(id).len()
+}
+
+fn mouse(kind: MouseEventKind, row: u16, col: u16) -> MouseEvent {
+    MouseEvent {
+        kind,
+        column: col,
+        row,
+        modifiers: KeyModifiers::NONE,
+    }
+}
+
+// ---------------------------------------------------------------------------
+// 1. Collapse (framing acceptance 1)
+// ---------------------------------------------------------------------------
+
+#[test]
+fn collapse_omits_hidden_lines_and_shifts_rows_up() {
+    let (s, _id) = seeded();
+    // Head = line 2, hidden = lines 3..=5.
+    fold_active(&s, 2, 5);
+    let cells = paint(&s);
+    let rows = text_rows(&cells);
+
+    assert_eq!(rows[0], "L00");
+    assert_eq!(rows[1], "L01");
+    assert_eq!(
+        rows[2], "L02 …",
+        "the head keeps its text plus the ellipsis"
+    );
+    // Lines 3..=5 are gone; the rows below shifted up.
+    assert_eq!(rows[3], "L06");
+    assert_eq!(rows[4], "L07");
+    assert_eq!(rows[5], "L08");
+    assert!(
+        !rows
+            .iter()
+            .any(|r| r.starts_with("L03") || r.starts_with("L04") || r.starts_with("L05")),
+        "no hidden line renders any row: {rows:?}"
+    );
+    // 12 source lines − 3 hidden = 9 content rows; row 9 is past the end.
+    assert_eq!(rows[8], "L11");
+    assert_eq!(
+        rows[9], "",
+        "content row count equals the visible-line count"
+    );
+}
+
+#[test]
+fn unfolded_frame_is_identical_to_the_pre_folding_baseline() {
+    // The `None` map path must be byte-identical, not merely equivalent.
+    let (s, _id) = seeded();
+    let baseline = paint(&s);
+    fold_active(&s, 2, 5);
+    let folded = paint(&s);
+    assert_ne!(baseline, folded, "the fixture actually folds");
+
+    let (s2, _id2) = seeded();
+    assert_eq!(baseline, paint(&s2), "no folds ⇒ unchanged rendering");
+}
+
+// ---------------------------------------------------------------------------
+// 2. Head marker in both gutter states (framing acceptance 2, round-1 F3)
+// ---------------------------------------------------------------------------
+
+#[test]
+fn head_marker_gutter_off_is_ellipsis_only() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    // Line numbers default to Off ⇒ gutter_width() == 0 ⇒ no sign cell.
+    let cells = paint(&s);
+    assert_eq!(row_text(&cells, 2), "L02 …");
+    assert_eq!(
+        at(&cells, 2, 0).glyph,
+        Glyph::Char('L'),
+        "with no gutter the text still starts at column 0 — no width change"
+    );
+}
+
+#[test]
+fn head_marker_gutter_on_adds_the_fold_glyph() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    exec(&s, "pmacs.window.set_line_numbers('absolute')");
+    let cells = paint(&s);
+    // 12 lines ⇒ 2 digits + 2 pad = 4-cell gutter; the fold glyph takes
+    // the leading pad cell, the number is right-aligned before the
+    // trailing pad, and the text begins at column 4.
+    assert_eq!(
+        at(&cells, 2, 0).glyph,
+        Glyph::Char('▸'),
+        "head row carries the gutter fold glyph"
+    );
+    assert_eq!(row_text(&cells, 2), "▸ 3 L02 …");
+    // A non-head row has no glyph.
+    assert_eq!(at(&cells, 1, 0).glyph, Glyph::Char(' '));
+    assert_eq!(row_text(&cells, 1), "  2 L01");
+}
+
+#[test]
+fn a_diagnostic_on_the_head_row_beats_the_fold_glyph() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    exec(&s, "pmacs.window.set_line_numbers('absolute')");
+    attach_diags(
+        &s,
+        vec![diag_on(pmacs::diag::DiagnosticSeverity::Error, 2, 2)],
+    );
+    let cells = paint(&s);
+    assert_eq!(
+        at(&cells, 2, 0).glyph,
+        Glyph::Char('E'),
+        "the diagnostic sign wins the shared cell (Q#FD20)"
+    );
+}
+
+// ---------------------------------------------------------------------------
+// 3. Line numbers (framing acceptance 3, Q#FD14)
+// ---------------------------------------------------------------------------
+
+#[test]
+fn absolute_numbers_skip_hidden_lines() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    exec(&s, "pmacs.window.set_line_numbers('absolute')");
+    let cells = paint(&s);
+    // Head shows 3, then the column jumps straight to 7 (line 6).
+    assert_eq!(row_text(&cells, 2), "▸ 3 L02 …");
+    assert_eq!(row_text(&cells, 3), "  7 L06");
+    assert_eq!(row_text(&cells, 4), "  8 L07");
+}
+
+#[test]
+fn relative_numbers_measure_visible_distance_across_a_fold() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    set_cursor(&s, 0); // cursor on line 0
+    exec(&s, "pmacs.window.set_line_numbers('relative')");
+    let cells = paint(&s);
+    // Visible order: 0, 1, 2(head), 6, 7 … so line 6 is 3 visible steps
+    // from the cursor line, not 6 raw lines.
+    assert_eq!(row_text(&cells, 0), "  0 L00");
+    assert_eq!(row_text(&cells, 1), "  1 L01");
+    assert_eq!(row_text(&cells, 2), "▸ 2 L02 …");
+    assert_eq!(row_text(&cells, 3), "  3 L06");
+    assert_eq!(row_text(&cells, 4), "  4 L07");
+}
+
+#[test]
+fn hybrid_shows_absolute_on_the_visible_cursor_row() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    // Cursor hidden on line 4 (a shared fold left it there).
+    set_cursor(&s, end_of(4));
+    exec(&s, "pmacs.window.set_line_numbers('hybrid')");
+    let cells = paint(&s);
+    // The anchor is the visible head (line 2), which shows absolute 3.
+    assert_eq!(row_text(&cells, 2), "▸ 3 L02 …");
+    assert_eq!(row_text(&cells, 1), "  1 L01");
+    assert_eq!(row_text(&cells, 3), "  1 L06", "one visible step below");
+}
+
+// ---------------------------------------------------------------------------
+// 4. Diagnostic clamp (framing acceptance 4, Q#FD15)
+// ---------------------------------------------------------------------------
+
+fn diag_on(
+    severity: pmacs::diag::DiagnosticSeverity,
+    line: u32,
+    end_col: u32,
+) -> pmacs::diag::Diagnostic {
+    pmacs::diag::Diagnostic {
+        start_line: line,
+        start_col: 0,
+        end_line: line,
+        end_col,
+        severity,
+        message: "boom".into(),
+        source: None,
+        code: None,
+    }
+}
+
+/// Attach the diagnostic overlay through the real Lua path and publish
+/// `diags` for the active buffer.
+fn attach_diags(state: &EditorState, diags: Vec<pmacs::diag::Diagnostic>) {
+    let uri: String = eval(
+        state,
+        r#"
+        local buf = pmacs.window.buffer()
+        local uri = "file:///tmp/folding_stage2_diag.rs"
+        assert(pmacs.diag._attach_view(buf, uri))
+        return uri
+        "#,
+    );
+    {
+        let core = state.core.borrow();
+        let registry = core.registry.clone();
+        let mut reg = registry.borrow_mut();
+        let buf = reg.get_mut(core.active_buffer_id()).unwrap();
+        buf.set_file_path(Some(std::path::PathBuf::from(
+            "/tmp/folding_stage2_diag.rs",
+        )));
+    }
+    state
+        .lsp_manager
+        .borrow()
+        .diag_store()
+        .lock()
+        .expect("diag store lock")
+        .set(uri, diags);
+}
+
+#[test]
+fn a_hidden_diagnostic_clamps_to_the_outermost_visible_head() {
+    let (s, _id) = seeded();
+    // Outer fold: head 0, hides 1..=9. Inner fold: head 3, hides 4..=6.
+    fold_active(&s, 0, 9);
+    fold_active(&s, 3, 6);
+    exec(&s, "pmacs.window.set_line_numbers('absolute')");
+    // A warning on the outer body and an ERROR on a NESTED inner line.
+    attach_diags(
+        &s,
+        vec![
+            diag_on(pmacs::diag::DiagnosticSeverity::Warning, 2, 3),
+            diag_on(pmacs::diag::DiagnosticSeverity::Error, 5, 3),
+        ],
+    );
+    let cells = paint(&s);
+    assert_eq!(
+        at(&cells, 0, 0).glyph,
+        Glyph::Char('E'),
+        "most-severe of the head and every line its fold hides, including \
+         a nested inner-fold line, surfaces on the OUTERMOST visible head"
+    );
+    // Nothing leaks onto the rows below the collapse.
+    assert_eq!(row_text(&cells, 1), " 11 L10");
+    assert_eq!(at(&cells, 1, 0).glyph, Glyph::Char(' '));
+}
+
+// ---------------------------------------------------------------------------
+// 5. Nested fold / shared cursor (framing acceptance 5, round-1 F1)
+// ---------------------------------------------------------------------------
+
+#[test]
+fn nested_fold_with_a_deeply_hidden_cursor_resolves_outermost() {
+    let (s, _id) = seeded();
+    fold_active(&s, 0, 9); // outer: hides 1..=9
+    fold_active(&s, 3, 6); // inner: head 3 is itself hidden
+    // A second frontend folded through the shared store while this
+    // window's logical cursor sat deep inside the nest.
+    set_cursor(&s, end_of(5));
+    exec(&s, "pmacs.window.set_line_numbers('relative')");
+
+    let caret = paint_caret(&s).expect("caret is on screen");
+    assert_eq!(caret.row, 0, "caret renders on the OUTERMOST visible head");
+
+    let cells = paint(&s);
+    assert_eq!(row_text(&cells, 0), "▸ 0 L00 …", "relative anchors there");
+    assert_eq!(row_text(&cells, 1), "  1 L10");
+}
+
+#[test]
+fn a_view_top_inside_a_nest_clamps_backward_to_the_head() {
+    let (s, _id) = seeded();
+    fold_active(&s, 0, 9);
+    fold_active(&s, 3, 6);
+    s.core.borrow_mut().active_window_mut().view_top = 5;
+    let cells = paint(&s);
+    assert_eq!(
+        view_top(&s),
+        0,
+        "clamped BACKWARD to the head, not forward past the fold"
+    );
+    assert_eq!(row_text(&cells, 0), "L00 …");
+}
+
+// ---------------------------------------------------------------------------
+// 6. Caret / selection / peer column projection
+//    (framing acceptance 6, round-2 F3 + round-3 F2)
+// ---------------------------------------------------------------------------
+
+#[test]
+fn a_hidden_caret_lands_at_the_heads_end_of_content_column() {
+    let s = editor();
+    let id = active_id(&s);
+    // Line 0 is deliberately SHORT and the hidden line is LONG, so a
+    // row-only clamp would leave the caret at a column the head does not
+    // even have.
+    seed(&s, id, "ab\nxxxxxxxxxxxxxxxxxxxx\ncd\nef\n");
+    let ok: bool = eval(
+        &s,
+        "return pmacs.fold.fold(pmacs.window.buffer(), { start = 2, ['end'] = 23 })",
+    );
+    assert!(ok);
+    // Cursor at column 15 of the hidden line 1.
+    set_cursor(&s, 3 + 15);
+    let caret = paint_caret(&s).expect("caret on screen");
+    assert_eq!(caret.row, 0, "the head row");
+    assert_eq!(
+        caret.col, 2,
+        "the head's end-of-content column, never the raw hidden column"
+    );
+}
+
+#[test]
+fn crossing_folds_project_a_point_to_the_first_visible_head() {
+    // Round-3 F2. A hides lines 1..=3 (head 0); B is headed on line 2 —
+    // itself hidden by A — and hides 3..=5. A point on line 5 is directly
+    // inside only B, whose `range.start` is hidden.
+    let (s, _id) = seeded();
+    fold_active(&s, 0, 3);
+    fold_active(&s, 2, 5);
+    assert_eq!(fold_count(&s), 2, "both crossing folds are stored");
+
+    set_cursor(&s, end_of(5));
+    let caret = paint_caret(&s).expect("caret on screen");
+    assert_eq!(caret.row, 0, "A's visible head, never B's hidden start");
+    assert_eq!(caret.col, 3, "L00's end-of-content column");
+
+    let cells = paint(&s);
+    let rows = text_rows(&cells);
+    assert_eq!(rows[0], "L00 …");
+    assert_eq!(rows[1], "L06", "lines 1..=5 all collapse under head 0");
+}
+
+#[test]
+fn a_selection_spanning_a_fold_paints_only_visible_rows() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    // Select from the middle of line 1 through the middle of line 7.
+    {
+        let mut core = s.core.borrow_mut();
+        let aw = core.active_window_mut();
+        aw.selection = Some(pmacs::window::Selection { anchor: 5 });
+        aw.cursor = end_of(7) - 1;
+    }
+    let cells = paint(&s);
+    let washed = |row: u32| {
+        (0..COLS)
+            .filter(|c| at(&cells, row, *c).style.reverse)
+            .count()
+    };
+    assert!(washed(1) > 0, "row 1 (L01) is partly selected");
+    assert!(washed(2) > 0, "row 2 (the visible head) is selected");
+    assert!(washed(3) > 0, "row 3 (L06, shifted up) is selected");
+    // Every painted row is a visible line; nothing renders for 3..=5 at
+    // all, so there is no hidden row to wash.
+    assert_eq!(row_text(&cells, 3), "L06");
+    assert!(washed(5) == 0, "past the selection end");
+}
+
+#[test]
+fn a_click_never_lands_on_a_hidden_line() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    let mut s = s;
+    // Grid row 3 shows L06 (line 6) after the collapse.
+    s.dispatch_mouse(
+        FrontendId::LOCAL,
+        mouse(
+            MouseEventKind::Down(crossterm::event::MouseButton::Left),
+            3,
+            0,
+        ),
+        CellSize::new(ROWS, COLS),
+    );
+    assert_eq!(cursor_line(&s), 6, "the 3rd VISIBLE line, not line 3");
+}
+
+#[test]
+fn a_peer_cursor_on_a_hidden_line_clamps_to_the_head_position() {
+    let (s, _id) = seeded();
+    let id = active_id(&s);
+    fold_active(&s, 2, 5);
+    let presence = pmacs::overlay_paint::OtherPresence {
+        frontend_id: FrontendId(2),
+        snapshot: pmacs::presence::PresenceSnapshot {
+            buffer_id: id,
+            cursor: end_of(4), // hidden line 4
+            selection: None,
+        },
+        color_slot: 0,
+    };
+    let mut backing = vec![Cell::default(); (ROWS * COLS) as usize];
+    let mut grid = CellGrid {
+        cells: &mut backing,
+        stride: COLS,
+        size: CellSize::new(ROWS, COLS),
+    };
+    pmacs::overlay_paint::paint_other_frontend_overlays(
+        &s,
+        &mut grid,
+        CellSize::new(ROWS, COLS),
+        &[presence],
+    );
+    // The peer paints on the head row at the head's end-of-content column.
+    assert!(
+        backing[(2 * COLS + 3) as usize].style.reverse,
+        "peer cursor clamped onto the visible head row/column"
+    );
+    for row in 3..6u32 {
+        assert!(
+            (0..COLS).all(|c| !backing[(row * COLS + c) as usize].style.reverse),
+            "nothing painted on a row a hidden line would have owned ({row})"
+        );
+    }
+}
+
+// ---------------------------------------------------------------------------
+// 7. Ordinary overlays across a fold (framing acceptance 7)
+// ---------------------------------------------------------------------------
+
+#[test]
+fn a_search_wash_across_a_fold_paints_only_visible_rows_correctly() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    let mut s = s;
+    // Search for "L0" — matches on lines 0..=9, several of them hidden.
+    ctrl(&mut s, 's');
+    type_str(&mut s, "L07");
+    press(&mut s, KeyCode::Enter);
+
+    let cells = paint(&s);
+    // L07 is on grid row 4 after the collapse (0,1,2,L06,L07).
+    assert_eq!(row_text(&cells, 4), "L07");
+    let washed_row_4 = (0..COLS)
+        .filter(|c| at(&cells, 4, *c).style != pmacs::cell::Style::default())
+        .count();
+    assert!(
+        washed_row_4 > 0,
+        "the match washes the row the folded frame actually put it on"
+    );
+    // Row 1 (L01) holds no match and stays unwashed.
+    assert!(
+        (0..COLS).all(|c| at(&cells, 1, c).style == pmacs::cell::Style::default()),
+        "no wash bleeds onto an unmatched visible row"
+    );
+}
+
+// ---------------------------------------------------------------------------
+// 8. Viewport / paging / indicator (framing acceptance 8, Q#FD18)
+// ---------------------------------------------------------------------------
+
+#[test]
+fn page_down_advances_by_a_screenful_of_visible_lines() {
+    let s = editor();
+    let id = active_id(&s);
+    let long = long_fixture();
+    seed(&s, id, &long);
+    // Hide lines 1..=40 under head 0.
+    let ok: bool = eval(
+        &s,
+        "return pmacs.fold.fold(pmacs.window.buffer(), { start = 4, ['end'] = 204 })",
+    );
+    assert!(ok);
+    let _ = paint(&s); // establish last_visible_rows
+    let before = cursor_line(&s);
+    assert_eq!(before, 0);
+    s.core.borrow_mut().move_page_down();
+    let after = cursor_line(&s);
+    assert!(
+        after > 40,
+        "a screenful of VISIBLE lines steps past the whole collapse (landed on {after})"
+    );
+    assert!(
+        !s.fold_registry
+            .folds(id)
+            .iter()
+            .any(|f| f.start < end_of_line_start(after) && end_of_line_start(after) <= f.end),
+        "the cursor never comes to rest on a hidden line"
+    );
+}
+
+/// Byte at the start of fixture-independent line `n` for a 5-byte line
+/// fixture (`"Mnnn\n"`).
+fn end_of_line_start(line: usize) -> u64 {
+    line as u64 * 5
+}
+
+#[test]
+fn the_mode_line_indicator_reckons_in_visible_lines() {
+    let (s, _id) = seeded();
+    // 12 lines in a 10-row window: not All.
+    let plain = paint(&s);
+    assert!(
+        row_text(&plain, 10).contains("Top"),
+        "unfolded 12 lines in 10 rows reads Top: {}",
+        row_text(&plain, 10)
+    );
+    // Collapse 3 lines away → 9 visible lines fit in 10 rows → All.
+    fold_active(&s, 2, 5);
+    let folded = paint(&s);
+    assert!(
+        row_text(&folded, 10).contains("All"),
+        "9 visible lines fit the viewport: {}",
+        row_text(&folded, 10)
+    );
+}
+
+#[test]
+fn goto_line_into_a_fold_leaves_its_head_visible() {
+    let s = editor();
+    let id = active_id(&s);
+    let long = long_fixture();
+    seed(&s, id, &long);
+    // Hide lines 51..=70 under head 50.
+    let ok: bool = eval(
+        &s,
+        "return pmacs.fold.fold(pmacs.window.buffer(), { start = 254, ['end'] = 354 })",
+    );
+    assert!(ok);
+    s.core.borrow_mut().move_to_line(60); // a hidden line
+    let _ = paint(&s);
+    let top = view_top(&s);
+    assert!(top <= 50, "view_top is at or above the head (got {top})");
+    assert!(
+        top + 10 > 50,
+        "and the head itself is inside the viewport (top {top})"
+    );
+    assert_eq!(fold_count(&s), 1, "goto-line does NOT auto-unfold");
+}
+
+// ---------------------------------------------------------------------------
+// 9. Vertical motion (framing acceptance 9, Q#FD17)
+// ---------------------------------------------------------------------------
+
+#[test]
+fn next_line_steps_across_a_collapsed_region_in_one_motion() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    let mut s = s;
+    set_cursor(&s, 0);
+    press(&mut s, KeyCode::Down);
+    assert_eq!(cursor_line(&s), 1);
+    press(&mut s, KeyCode::Down);
+    assert_eq!(cursor_line(&s), 2, "the head");
+    press(&mut s, KeyCode::Down);
+    assert_eq!(cursor_line(&s), 6, "one step clears the whole collapse");
+    press(&mut s, KeyCode::Up);
+    assert_eq!(cursor_line(&s), 2, "and one step back returns to the head");
+}
+
+#[test]
+fn motion_from_a_hidden_cursor_normalizes_to_the_visible_head_first() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    let mut s = s;
+    // A shared fold left the logical cursor deep inside the collapse.
+    set_cursor(&s, end_of(4));
+    press(&mut s, KeyCode::Down);
+    assert_eq!(
+        cursor_line(&s),
+        6,
+        "normalize to head 2, then step to the next visible line"
+    );
+
+    set_cursor(&s, end_of(4));
+    press(&mut s, KeyCode::Up);
+    assert_eq!(
+        cursor_line(&s),
+        1,
+        "normalize to head 2, then step to the previous visible line"
+    );
+}
+
+// ---------------------------------------------------------------------------
+// 10. Interactive unfold widening (framing acceptance 10, Q#FD19)
+// ---------------------------------------------------------------------------
+
+#[test]
+fn yank_at_a_point_inside_a_fold_unfolds_it() {
+    let (s, _id) = seeded();
+    let mut s = s;
+    // Kill line 0's text so the kill ring has content.
+    set_cursor(&s, 0);
+    ctrl(&mut s, 'k');
+    // Re-fold with the shortened buffer: head 2, hidden 3..=5 of the
+    // remaining text. Recompute from the live line index.
+    let (head_end, last_end) = line_ends(&s, 2, 5);
+    fold_range(&s, head_end, last_end);
+    assert_eq!(fold_count(&s), 1);
+
+    // Put point INSIDE the fold and yank.
+    set_cursor(&s, last_end);
+    ctrl(&mut s, 'y');
+    assert_eq!(
+        fold_count(&s),
+        0,
+        "yank at a point inside a fold unfolds it"
+    );
+}
+
+#[test]
+fn query_replace_inside_a_fold_unfolds_it() {
+    let (s, _id) = seeded();
+    let mut s = s;
+    fold_active(&s, 2, 5);
+    assert_eq!(fold_count(&s), 1);
+    set_cursor(&s, 0);
+    // M-% L04 RET Q04 RET, then `y` to replace the first match — which
+    // is on hidden line 4.
+    alt(&mut s, '%');
+    type_str(&mut s, "L04");
+    press(&mut s, KeyCode::Enter);
+    type_str(&mut s, "Q04");
+    press(&mut s, KeyCode::Enter);
+    type_str(&mut s, "y");
+    assert_eq!(
+        fold_count(&s),
+        0,
+        "the replacement's edit point was inside the fold"
+    );
+}
+
+/// Define a command whose body is `body`, then run it the way a user
+/// does — `M-x <name> RET`. The **Rust** dispatch path is what installs
+/// the `InteractiveCommandOrigin` scope (`pmacs.command.invoke_interactive`
+/// alone does not), so the widening must be driven through it.
+fn define(s: &EditorState, name: &str, body: &str) {
+    exec(
+        s,
+        &format!(
+            "pmacs.command.define {{ name = '{name}', \
+             description = 'stage 2 test', fn = function() {body} end }}"
+        ),
+    );
+}
+
+fn m_x(s: &mut EditorState, name: &str) {
+    alt(s, 'x');
+    type_str(s, name);
+    press(s, KeyCode::Enter);
+}
+
+#[test]
+fn an_interactive_lua_mutator_unfolds_but_a_programmatic_one_does_not() {
+    let (s, _id) = seeded();
+    let mut s = s;
+    fold_active(&s, 2, 5);
+    set_cursor(&s, end_of(4)); // point inside the fold
+
+    // A PROGRAMMATIC data-API edit: no interactive command in scope.
+    // (Insert at 0 is before the fold, so it only translates it right.)
+    exec(&s, "pmacs.window.buffer():insert(0, 'x')");
+    assert_eq!(
+        fold_count(&s),
+        1,
+        "a bare data-API mutation stays programmatic — no unfold"
+    );
+
+    // The same call from INSIDE an interactive command unfolds.
+    define(&s, "test.poke", "pmacs.window.buffer():insert(0, 'y')");
+    set_cursor(&s, end_of(4) + 1); // still inside the shifted fold
+    m_x(&mut s, "test.poke");
+    assert_eq!(
+        fold_count(&s),
+        0,
+        "an interactive command's edit at the point unfolds (Q#FD19)"
+    );
+}
+
+#[test]
+fn a_bypass_intercept_interactive_edit_also_unfolds() {
+    // Pins the seam at `run_buffer_edit`, above the managed/bypass split:
+    // hooking only `run_managed_edit` would let this one escape.
+    let (s, _id) = seeded();
+    let mut s = s;
+    fold_active(&s, 2, 5);
+    set_cursor(&s, end_of(4));
+    define(
+        &s,
+        "test.bypass",
+        "pmacs.window.buffer():insert(0, 'z', { bypass_intercept = true })",
+    );
+    m_x(&mut s, "test.bypass");
+    assert_eq!(
+        fold_count(&s),
+        0,
+        "a bypass_intercept interactive edit must not escape the widening"
+    );
+}
+
+#[test]
+fn an_interactive_edit_to_an_inactive_buffer_does_not_unfold() {
+    let (s, _id) = seeded();
+    let mut s = s;
+    fold_active(&s, 2, 5);
+    set_cursor(&s, end_of(4));
+    // A second, NOT-displayed buffer; the command edits that one.
+    exec(
+        &s,
+        "other = pmacs.buffer.from_bytes('other.txt', 'aaa\\nbbb\\n')",
+    );
+    define(&s, "test.other", "other:insert(0, 'q')");
+    m_x(&mut s, "test.other");
+    assert_eq!(
+        fold_count(&s),
+        1,
+        "an explicit inactive-buffer mutation stays programmatic"
+    );
+}
+
+#[test]
+fn undo_and_redo_do_not_unfold() {
+    // Explicitly DEFERRED in the framing (round-1 F5 ruling); pinned so
+    // the deferral is a decision, not an accident.
+    let (s, _id) = seeded();
+    let mut s = s;
+    set_cursor(&s, end_of(7));
+    type_str(&mut s, "Z"); // an edit to undo, outside any fold
+    fold_active(&s, 2, 5);
+    set_cursor(&s, end_of(4));
+    ctrl(&mut s, '_'); // undo
+    assert_eq!(fold_count(&s), 1, "undo does not unfold (deferred)");
+}
+
+/// Content-end bytes of the current buffer's lines `a` and `b`.
+fn line_ends(s: &EditorState, a: usize, b: usize) -> (u64, u64) {
+    let core = s.core.borrow();
+    let registry = core.registry.clone();
+    let reg = registry.borrow();
+    let buf = reg.get(core.active_buffer_id()).unwrap();
+    let tv = &core.active_window().text_view;
+    let end = |line: usize| tv.line_offset(line).unwrap() + tv.line_len(buf, line).unwrap();
+    (end(a), end(b))
+}
+
+fn fold_range(s: &EditorState, start: u64, end: u64) {
+    let ok: bool = eval(
+        s,
+        &format!(
+            "return pmacs.fold.fold(pmacs.window.buffer(), {{ start = {start}, ['end'] = {end} }})"
+        ),
+    );
+    assert!(ok, "fold({start}..{end}) must be accepted");
+}
+
+// ---------------------------------------------------------------------------
+// 11. No wire / protocol change (framing acceptance 11, Bet B6)
+// ---------------------------------------------------------------------------
+
+#[test]
+fn stage_2_bumps_no_protocol_version() {
+    assert_eq!(
+        pmacs::protocol::PROTOCOL_VERSION,
+        19,
+        "Stage 2 is entirely daemon-side: the TUI collapse ships no new wire data"
+    );
+    assert_eq!(
+        *pmacs::protocol::SUPPORTED_PROTOCOL_VERSIONS.last().unwrap(),
+        19
+    );
+}
+
+// ---------------------------------------------------------------------------
+// 12. Shared store, independent viewports (framing acceptance 12)
+// ---------------------------------------------------------------------------
+
+#[test]
+fn two_windows_on_one_buffer_both_collapse_with_their_own_view_tops() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    exec(&s, "pmacs.window.split_horizontal()");
+    // Give the second window a different view_top (visible line 2 = head).
+    let ids: Vec<WindowId> = s.core.borrow().windows.keys().copied().collect();
+    assert_eq!(ids.len(), 2, "the split produced two windows");
+    s.core
+        .borrow_mut()
+        .windows
+        .get_mut(&ids[1])
+        .unwrap()
+        .view_top = 1;
+
+    let cells = paint(&s);
+    // 12 rows: status row 11; two stacked windows of 5 and 6 rows, each
+    // with a mode line. Window A text rows 0..=3, window B text rows
+    // 5..=9 (its own mode line last).
+    assert_eq!(row_text(&cells, 0), "L00");
+    assert_eq!(row_text(&cells, 2), "L02 …", "window A collapses");
+    let b_rows: Vec<String> = (5..10).map(|r| row_text(&cells, r)).collect();
+    assert!(
+        b_rows.iter().any(|r| r == "L02 …"),
+        "window B collapses too, from its own view_top: {b_rows:?}"
+    );
+    assert!(
+        !b_rows.iter().any(|r| r.starts_with("L03")),
+        "no hidden line renders in the second window either: {b_rows:?}"
+    );
+    // Both windows keep their own view_top.
+    assert_eq!(s.core.borrow().windows[&ids[1]].view_top, 1);
+}
+
+// ---------------------------------------------------------------------------
+// 13. Frontend-scoped motion (framing acceptance 13, round-2 F1 / Q#FD21)
+// ---------------------------------------------------------------------------
+
+/// Register a second frontend on the SAME buffer, with an explicit
+/// projection choice — the attach-time decision the daemon makes from the
+/// negotiated selected-render bit.
+fn attach_frontend(s: &EditorState, fid: FrontendId, fold_projection: bool) -> WindowId {
+    let mut core = s.core.borrow_mut();
+    let buffer_id = core.active_buffer_id();
+    let text_view = {
+        let registry = core.registry.clone();
+        let reg = registry.borrow();
+        pmacs::text_view::TextView::new(reg.get(buffer_id).unwrap())
+    };
+    let win_id = WindowId::next();
+    core.windows
+        .insert(win_id, Window::new(win_id, buffer_id, text_view));
+    core.register_frontend_view(
+        fid,
+        FrontendView {
+            layout: Layout::single(win_id),
+            active: win_id,
+            fold_projection,
+        },
+    );
+    win_id
+}
+
+/// Move down once as `fid` and report that frontend's resulting line.
+fn move_down_as(s: &EditorState, fid: FrontendId, win_id: WindowId) -> usize {
+    let mut core = s.core.borrow_mut();
+    core.active_frontend = fid;
+    core.move_down();
+    let win = &core.windows[&win_id];
+    win.text_view.line_at_offset(win.cursor)
+}
+
+fn move_up_as(s: &EditorState, fid: FrontendId, win_id: WindowId) -> usize {
+    let mut core = s.core.borrow_mut();
+    core.active_frontend = fid;
+    core.move_up();
+    let win = &core.windows[&win_id];
+    win.text_view.line_at_offset(win.cursor)
+}
+
+#[test]
+fn a_semantic_frontend_keeps_raw_line_motion_while_the_tui_folds() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    // A grid session and a semantic session on the same buffer, sharing
+    // the fold store (daemon.rs: a frontend holds exactly one of a
+    // RenderState or a SemanticRenderState; both may attach at once).
+    let grid_win = attach_frontend(&s, FrontendId(2), true);
+    let sem_win = attach_frontend(&s, FrontendId(3), false);
+    for win in [grid_win, sem_win] {
+        s.core.borrow_mut().windows.get_mut(&win).unwrap().cursor = end_of(2);
+    }
+
+    assert_eq!(
+        move_down_as(&s, FrontendId(2), grid_win),
+        6,
+        "the grid session steps by VISIBLE lines, skipping the collapse"
+    );
+    assert_eq!(
+        move_down_as(&s, FrontendId(3), sem_win),
+        3,
+        "the semantic session still DISPLAYS line 3, so it must land there"
+    );
+    assert_eq!(
+        move_up_as(&s, FrontendId(3), sem_win),
+        2,
+        "and steps back by raw lines too"
+    );
+}
+
+#[test]
+fn semantic_paging_stays_raw_while_the_grid_pages_visibly() {
+    let s = editor();
+    let id = active_id(&s);
+    let long = long_fixture();
+    seed(&s, id, &long);
+    // Hide lines 1..=40 under head 0.
+    let ok: bool = eval(
+        &s,
+        "return pmacs.fold.fold(pmacs.window.buffer(), { start = 4, ['end'] = 204 })",
+    );
+    assert!(ok);
+    let grid_win = attach_frontend(&s, FrontendId(2), true);
+    let sem_win = attach_frontend(&s, FrontendId(3), false);
+
+    let page_as = |fid: FrontendId, win: WindowId| {
+        let mut core = s.core.borrow_mut();
+        core.active_frontend = fid;
+        core.move_page_down();
+        let w = &core.windows[&win];
+        w.text_view.line_at_offset(w.cursor)
+    };
+    let grid_line = page_as(FrontendId(2), grid_win);
+    let sem_line = page_as(FrontendId(3), sem_win);
+    assert!(
+        grid_line > 40,
+        "a grid page clears the whole collapse (landed on {grid_line})"
+    );
+    assert!(
+        sem_line <= 40,
+        "a semantic page counts the lines it still shows (landed on {sem_line})"
+    );
+}
+
+#[test]
+fn detaching_the_semantic_frontend_leaves_the_grid_unaffected() {
+    let (s, _id) = seeded();
+    fold_active(&s, 2, 5);
+    let grid_win = attach_frontend(&s, FrontendId(2), true);
+    let sem_win = attach_frontend(&s, FrontendId(3), false);
+    s.core
+        .borrow_mut()
+        .windows
+        .get_mut(&sem_win)
+        .unwrap()
+        .cursor = end_of(2);
+    assert_eq!(move_down_as(&s, FrontendId(3), sem_win), 3);
+
+    s.core.borrow_mut().unregister_frontend_view(FrontendId(3));
+    s.core
+        .borrow_mut()
+        .windows
+        .get_mut(&grid_win)
+        .unwrap()
+        .cursor = end_of(2);
+    assert_eq!(
+        move_down_as(&s, FrontendId(2), grid_win),
+        6,
+        "the flag is per-FrontendView — the grid never inherited it"
+    );
+}
+
+// ---------------------------------------------------------------------------
+// 14. Split of different buffers, one folded (framing acceptance 14,
+//     round-2 F2 + round-3 F1)
+// ---------------------------------------------------------------------------
+
+/// A vertical split showing buffer A (folded, active) beside buffer B
+/// (unfolded). Returns `(a_window, b_window)`.
+fn split_two_buffers(s: &EditorState) -> (WindowId, WindowId) {
+    let a_win = s.core.borrow().active_window_id();
+    exec(s, "pmacs.window.split_vertical()");
+    let ids: Vec<WindowId> = s.core.borrow().windows.keys().copied().collect();
+    let b_win = *ids.iter().find(|w| **w != a_win).expect("second window");
+    // Point B at a different buffer.
+    exec(
+        s,
+        "other = pmacs.buffer.from_bytes('other.txt', 'B0\\nB1\\nB2\\nB3\\nB4\\nB5\\nB6\\nB7\\n')",
+    );
+    let other: BufferId = {
+        let core = s.core.borrow();
+        let registry = core.registry.clone();
+        let reg = registry.borrow();
+        reg.find_by_name("other.txt").expect("other.txt present")
+    };
+    let text_view = {
+        let core = s.core.borrow();
+        let registry = core.registry.clone();
+        let reg = registry.borrow();
+        pmacs::text_view::TextView::new(reg.get(other).unwrap())
+    };
+    let mut core = s.core.borrow_mut();
+    let w = core.windows.get_mut(&b_win).unwrap();
+    w.buffer_id = other;
+    w.text_view = text_view;
+    w.cursor = 0;
+    drop(core);
+    s.core.borrow_mut().set_active_window_id(a_win);
+    (a_win, b_win)
+}
+
+#[test]
+fn an_unfolded_pane_beside_a_folded_one_is_byte_identical_to_its_baseline() {
+    let (s, _id) = seeded();
+    let (_a_win, b_win) = split_two_buffers(&s);
+    let baseline = paint(&s);
+
+    // Now fold buffer A only.
+    fold_active(&s, 2, 5);
+    let folded = paint(&s);
+
+    // Window A collapsed …
+    let a_rows: Vec<String> = (0..TEXT_ROWS).map(|r| row_text(&folded, r)).collect();
+    assert!(
+        a_rows.iter().any(|r| r.contains("L02 …")),
+        "the folded pane collapsed: {a_rows:?}"
+    );
+    // … while every cell of window B is unchanged. B occupies the right
+    // half of a vertical split.
+    let b_origin = COLS / 2;
+    for row in 0..TEXT_ROWS {
+        for col in b_origin..COLS {
+            assert_eq!(
+                at(&baseline, row, col),
+                at(&folded, row, col),
+                "buffer B's window leaked A's fold map at ({row},{col})"
+            );
+        }
+    }
+    assert_eq!(s.core.borrow().windows[&b_win].view_top, 0);
+}
+
+#[test]
+fn wheel_over_an_inactive_unfolded_pane_uses_that_windows_map() {
+    let (s, _id) = seeded();
+    let (a_win, b_win) = split_two_buffers(&s);
+    fold_active(&s, 2, 5); // buffer A (active) folds; B does not
+    let mut s = s;
+    let _ = paint(&s);
+
+    // Wheel down over the RIGHT half — buffer B's pane, which the wheel
+    // does NOT activate.
+    s.dispatch_mouse(
+        FrontendId::LOCAL,
+        mouse(MouseEventKind::ScrollDown, 1, (COLS / 2 + 2) as u16),
+        CellSize::new(ROWS, COLS),
+    );
+
+    assert_eq!(
+        s.core.borrow().active_window_id(),
+        a_win,
+        "a wheel event does not move focus"
+    );
+    let b_top = s.core.borrow().windows[&b_win].view_top;
+    assert_eq!(
+        b_top, 3,
+        "B scrolled by raw lines through ITS OWN map (SCROLL_LINES), \
+         not through the folded active buffer's"
+    );
+    assert_eq!(
+        s.core.borrow().windows[&a_win].view_top,
+        0,
+        "the folded pane did not scroll"
+    );
+}
diff --git a/tests/m4_acceptance.rs b/tests/m4_acceptance.rs
index 69c8500..ca44980 100644
--- a/tests/m4_acceptance.rs
+++ b/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,
diff --git a/tests/statusline_segments_acceptance.rs b/tests/statusline_segments_acceptance.rs
index 8d6f1d2..2963fea 100644
--- a/tests/statusline_segments_acceptance.rs
+++ b/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,
             },
         );
     }
diff --git a/tests/tab_width_acceptance.rs b/tests/tab_width_acceptance.rs
index ba15f52..e153983 100644
--- a/tests/tab_width_acceptance.rs
+++ b/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,
     }
 }
 
diff --git a/tests/vterm_stage3_acceptance.rs b/tests/vterm_stage3_acceptance.rs
index 4a170af..8d7eb46 100644
--- a/tests/vterm_stage3_acceptance.rs
+++ b/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