feat(review): collapse and expand files like a GitHub diff

[narthur]

Why

Long agent changesets mix files you still need to read with ones you've already reviewed, and there was no way to set a file aside while scrolling the stream. This adds GitHub-style per-file collapse so you can fold away what you're done with.

This is the manual, user-driven counterpart to the prior collapse attempts, designed to sidestep the feedback they ran into:

• feat(review): mark hunks as reviewed with per-repo persistence #421 (v mark-reviewed, per-repo marker store) — reviewers flagged disk persistence written for non-repo inputs, path traversal on the hash filename, and synchronous disk I/O blocking the first render, plus a "what's the prior art for v?" question. Avoided here: collapse is session-only in-memory state (reconciled across watch reloads exactly like the existing gap-expansion state), so there is no marker store, repo-root gating, hash validation, or TTL to get wrong.
• feat(review): collapse lockfiles, minified, and generated files by default #470 (auto-collapse generated/lockfiles) — reviewers flagged regex/perf bugs in the content-classification heuristics. Avoided here: there is no classification; collapse is purely a manual toggle. This reuses feat(review): collapse lockfiles, minified, and generated files by default #470's blessed architecture (collapse = swap in a zero-hunk placeholder variant at the derivation layer) without its detection surface.
• feat(ui): mark files to hide them from the review stream #277 (m hide files) — different intent (hide vs. collapse); its one note was a click-fallthrough guard, which we mirror by stopping propagation on the chevron handler.

User-visible

• x collapses/expands the selected file.
• Shift+X collapses or expands every file.
• Clicking the chevron (▾/▸) in any file header toggles that file; clicking the rest of the header still selects/jumps as before.
• A collapsed file shows just its header plus a one-line "Collapsed" placeholder; sidebar stats are unchanged.
• Help dialog and the Navigate menu document the new controls.

Collapse state is session-only and resets on relaunch (it survives watch-mode reloads).

What it does

• A collapsed file is swapped for a zero-hunk placeholder variant in buildReviewState (src/ui/lib/fileCollapse.ts), so the existing empty-file render (PierreDiffView), geometry (bodyHeight: 1), and hunk-cursor paths handle it with no new renderer flags or row kinds. Hunk navigation ([/]) naturally skips collapsed files because they expose no hunks.
• useReviewController owns a collapsedFileIds set with toggleFileCollapsed / toggleAllFilesCollapsed, reconciled in the same filesSnapshot block as gap expansion. Toggling re-pins the file header to the top so a height change can't scroll it out of view.
• The placeholder variant is cached per source file in a WeakMap so its object identity stays stable across renders (the geometry cache keys on the DiffFile object).

Tests

• src/ui/lib/fileCollapse.test.ts — variant swap empties hunks while preserving stats/identity, returns a stable object, swaps only collapsed ids, and prunes ids that left the changeset.
• src/ui/hooks/useReviewController.test.tsx — toggling collapses to a zero-hunk variant, skips hunk navigation, expands back, and collapse-all/expand-all round-trip.
• test/pty/collapse.test.ts — real PTY: x collapses the file to the placeholder (with the ▸ chevron) and x again restores the diff.
• Updated AppHost.interactions, ui-components (Help dialog), and ui-lib (menus) for the chevron and new entries.
• bun run typecheck, bun run lint, and bun run test:tty-smoke clean.

🤖 Generated with Claude Code

[greptile-apps]

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