Two small revisions, and every Stage 4 question is now answered.
Approval is NOT recorded; no implementation may begin.
Q#HS7(c″) — the tab-straddle mapping. Recorded as PRESERVED rather
than chosen, because it already exists: `byte_at_place`'s doc comment
says it rounds forward to the next character boundary, and the walk
accumulates past the tab byte and returns on the NEXT character's
start column, so every column inside a tab's expansion already yields
the post-tab offset (src/text_view.rs:224, :243-254). The requirement
on Stage 4 is therefore that horizontal scroll not PERTURB it — which
makes its witness a regression test, and one that should fail if the
walk is ever "optimized" to start at the effective edge instead of
column 0.
The obvious objection is that (c′) rounds backward and (c″) forward,
so the framing answers it. A wide glyph's two cells belong to ONE
character: forward-rounding its trailing cell would designate it to
the next character and leave the straddling glyph with no visible cell
mapping to it at all — unreachable by click exactly when it is what
the user scrolled toward. A tab's expansion cells are whitespace
BETWEEN the tab byte and the next character, and forward-rounding them
is already how clicking in indentation lands at the start of the text.
Different directions, one principle: every visible cell is designated
to the byte a user would mean by clicking it.
With (c′) and (c″) the (d) contract is total over visible cells:
ordinary character → its own start; bisected wide glyph → the glyph's
start; tab expansion → the byte after the tab.
Q#HS5 approved as stated, with both conditions written into the
approval rather than attached as advice: `#[serde(default)]` and a
literal v1 JSON fixture omitting the field, asserting restore at zero.
The handoff said "Stage 4 is the remainder". It now says Stages 4 AND
5 remain and the arc closes at Stage 5, carries the Q#HS1 time box,
and states explicitly that Rule 4 must not retire the long-lines lane
at Stage 4's merge. It also records that the unreachable caveat is
missing from the setting's description — the #221 gap — so that fact
lives in the durable doc and not only in a lane block that will
eventually be removed.
Ledger: the question list is consolidated (the accepted answers had
begun duplicating the blocking entries they resolved), keeping the
withdrawn (c)'s reasoning because the trap generalizes to any future
window-wide value derived from per-line content.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016bqGA6s9tTUFzYpbeW3tai
Levi Neuwirth
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