fix: land the selection-padding highlight on the dragged cells

[Sma1lboy]

Direction

Bugs / correctness — a self-found rendering bug in the embedded terminal's grid selection, surfaced by a review pass over the pure/near-pure logic modules. (Open issues #362/#307/#259 were either awaiting an owner design decision or too broad for one tightly-scoped slice; this is a smaller, fully-verifiable fix.)

Problem

In terminal-selection.ts, overlayRowSpan paints a selection's inverse-video highlight over a row. Snapshot rows are trimmed, not grid-padded (xtermLineToChunks drops trailing blank cells), while the mouse column is clamped to the grid width — so a drag can anchor in the blank padding to the right of a short line, giving a span whose start column from is greater than the row's painted width col.

The trailing-padding branch computed the highlight width as to - Math.max(col, from) but appended it immediately after the painted content (at visual column col). When from > col, the block rendered at [col, col + (to - from)) instead of the intended [from, to) — the highlight was shifted left onto cells the user never dragged over, the gap [col, from) was wrongly inverted, and the row came out too short.

Concrete case (grid width 8, row "ab", select cols 5–7): expected inverse on cols 5/6/7 with cols 2–4 left plain; actual output inverted cols 2/3/4 and produced a 5-cell row. The copied text was already correct (extractSelection slices the trimmed text independently) — only the on-screen highlight was misplaced.

Fix

When the span starts past the painted cells, emit the from - col gap as plain spaces first, then the inverse block sized to - Math.max(col, from). When from <= col the gap is zero and the inverse block fills [col, to) exactly as before, so the common case is byte-identical.

Verification

• Added a cell-exact test asserting the highlight lands on the selected padding cells (cols 5–7) with the gap (cols 2–4) left unhighlighted — this fails against the old code, which produced "ab " with the highlight on cols 2–4.
• bun test test/tui/terminal-selection.test.ts → 13/13 pass.
• bun run lint and bun run typecheck → both green.
• Full test/tui/ suite: clean tree 842 pass / 43 fail vs. this branch 843 pass / 43 fail — exactly the one new passing test and zero new failures. (The 43 pre-existing failures are unrelated environment issues in the CI-style checkout — readonly process.platform / Bun-global reassignment in bun's test runner.)

Follow-ups deferred

• The engine/foreground.ts custom-engine gate matches basename(argv0) against extraBin without the binaryName() normalization the builtin path uses (could false-negative on aider.exe), and handlers-inspect.ts filters sessions by bare startsWith(taskId) rather than `${taskId}::` — both are lower-confidence and left for a separate, scoped change.

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
kobe-landing	Ignored		Aug 12, 2026 4:38pm

[claude]

Claude finished @Sma1lboy's task in 3m 5s —— View job

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Verdict: correct, minimal fix with a valid regression test — one house-rule attribution cleanup needed before merge.

I traced overlayRowSpan's new branch by hand and ran it through code-reviewer, silent-failure-hunter, pr-test-analyzer, type-design-analyzer, comment-analyzer, and code-simplifier. All confirm the arithmetic is sound: when from > col, the emitted row is col + (from-col) + (to-from) = to with the inverse block landing exactly on [from, to) (previously it landed on [col, col+(to-from)), shifted left by from-col — the reported bug). When from <= col, gap is 0 and the output is byte-identical to the old path, so the common case is unaffected.

Blocking

• AI attribution in the PR body and commit author, violating CLAUDE.md's non-negotiable rule ("NEVER add ... 'Generated with Claude Code' footers"). The PR description ends with _Generated by [Claude Code](https://claude.ai/code/session_...)_, and the landed commit (9698253) is authored as Claude <noreply@anthropic.com> rather than a human identity. Neither should carry AI attribution per the repo's hard rules — please strip the footer from the PR body before merge, and if the PR is squash-merged, make sure the squash commit doesn't inherit the author/footer.

Should-fix

None — the fix itself, its scope, and its test coverage are sound.

Nits

• terminal-selection.ts:141-147 — the comment is ~7 lines for a 4-line arithmetic change; code-reviewer and code-simplifier both flagged it as more verbose than the fix needs (2-3 sentences would cover the what/why).
• terminal-selection.ts:148-152 — gap and Math.max(col, from) re-derive the same from > col comparison twice; code-simplifier's suggested rewrite (gap = Math.max(0, from - col), then to - col - gap for the inverse length) is equivalent and avoids the duplication, optional.
• test/tui/terminal-selection.test.ts:91 — expect(displayWidth(...)).toBe(8) is redundant with the string-equality assertion on the line above ("ab " is already 8 cells of plain ASCII).

Other checks (all clean)

• Regression test: pr-test-analyzer confirmed the new case (test/tui/terminal-selection.test.ts:81-102) fails against pre-fix code (would produce a 5-cell row with no plain gap) and passes post-fix — a genuine pin, not implementation-mirroring.
• File size cap: terminal-selection.ts is 172 lines, well under the 500-line cap.
• Changeset: present, correctly scoped patch, matches the PR's stated intent.
• Silent failures / invariants: no swallowed errors; rowSpan guarantees from < to so no negative-width .repeat() calls; gap is guarded by gap > 0.
• Type design / comments: no new types needed for this logic-only change; the (verbose) explanatory comments accurately describe the code, no drift.
• Not applicable to this diff: keybindings, flex layout, engine-owned UI data.