Permanent editor-style scrollbar on the Changes dock; static hover rows (upstream 6003ae11, 84d4fa18)

[evilchinesefood]

Part of #417 (upstream main after v0.11.0). Upstream commits 6003ae11 feat(changes): add a permanent editor-style scrollbar (9 files, +227/-44) and 84d4fa18 perf(diff): use static hover backgrounds in the diff sidebar rows (5 lines). Small dock polish; the second is a fork UX decision.

What upstream did

• Permanent scrollbar: the Changes dock gets an always-visible Zed-style vertical scrollbar (15 px track, 25 px minimum thumb, 1 px left border, hover and drag tints) in its own gutter beside #diff-dock-scroll (min_w_0()), with centred track clicks and dragging. New widgets/scrollbar/geometry.rs (SCROLLBAR_SIZE, MIN_THUMB, Track, scrollbar_track, extracted from code/navigation/layout.rs of Zed minimap and scrollbar controls in dock file tabs (upstream e31cf649) #435) and widgets/editor_scrollbar.rs::EditorScrollbar (canvas paint, capture-phase mouse move/up, Rc<Cell<_>> state); DiffDockState.vertical_scrollbar. No tests.
• Static hover: Review changed-files rows (file, directory, branch group) swap .animated_hover_bg(resting, hover) for .hover(|s| s.bg(hover)) at three sites and drop the AnimatedHoverExt imports, so a hover no longer schedules a fade animation and its redraws.

What the fork has

• No widgets/editor_scrollbar.rs, no widgets/scrollbar/ directory; widgets/scrollbar.rs is the div-overlay design (:1-40, SCROLLBAR_MIN_THUMB = 24 at :160). No code/navigation/ (Zed minimap and scrollbar controls in dock file tabs (upstream e31cf649) #435 not ported). DiffDockState at main.rs:1139; dock scroll host at mod.rs:619-650 (h_scroll_drag, fix(fork): Changes dock displays Git usage errors for folders without a repository #393 blank-dock branch).
• Fork still animates: diff_sidebar/mod.rs:18,434,524, rows.rs:8,90; the primitive is fork-authored (ui_primitives.rs:198-244, 7a8ab68a), and 10fbde55 reshaped these rows, so line numbers differ but the sites map 1:1.

Port

git show 6003ae11 84d4fa18

• Without Zed minimap and scrollbar controls in dock file tabs (upstream e31cf649) #435, port geometry.rs fresh and drop the navigation/layout.rs hunk. Two scrollbar systems will coexist until Zed minimap and scrollbar controls in dock file tabs (upstream e31cf649) #435 lands; say so in widgets/mod.rs.
• Wire vertical_scrollbar into the fork's mod.rs:619-650 host, keeping the fix(fork): Changes dock displays Git usage errors for folders without a repository #393 blank-dock branch and h_scroll_drag.
• 84d4fa18: decide whether the fork keeps its animated hover on those rows (it is a deliberate fork choice). If dropped, drop it consistently with the file rows Files tree on a worker thread with cached snapshots (upstream d6a44bfc) #430 touches.

Done when

• Changes dock shows a permanent right-hand scrollbar; click on the track centres the thumb there; drag scrolls; the thumb never shrinks below 25 px; the horizontal drag (h_scroll_drag) still works and the empty-repo dock stays blank (fix(fork): Changes dock displays Git usage errors for folders without a repository #393).
• Six gates green.

[evilchinesefood]

[issues-fix] Working on this (run claude-20260906-110613, started 2026-09-06). A pull request follows from this session.

[evilchinesefood]

[issues-fix] Fixed in PR #447 (branch feat/mcp-nudge-pane-cache-dock-scrollbar); the issue closes when the PR merges.