Reader-driven paging, keyboard travel, and no dead height in the Commit log

Traceability

• Spec ID: studio-commit-log-paging-and-keys
• Status: implemented

Intent

Three defects were reported against the Commits View and reproduced against a local Studio (better-harness, 505 reachable commits, 15 of them authored today).

1. Paging looks dead, because it already ran to exhaustion.CommitTable puts a 1px sentinel in the scroll container after the virtualized rows and loads the next page whenever that sentinel intersects, with a 160px bottom rootMargin. The rendered rows are datedCommits — the loaded history narrowed by the sidebar date window — while a page is fetched from the unnarrowed history. So when the window admits few commits the sentinel never leaves the viewport, every appended page re-arms the observer, and the View drains the whole history in a burst with no reader input.

Measured, entering Commits and switching the window to Today: 10 back-to-back /api/git/log requests (offsets 120…480), 505 commits loaded, 15 rendered. After that hasMore is false, so scrolling can never load anything again — which is exactly what "infinite loading does not work" looks like from the reader's side. With the window on All time the list overflows, the sentinel sits below the fold, and paging behaves correctly (1 request on entry, 1 per scroll to the end).

The trigger is therefore wrong in both directions: it fires when the reader has not asked for more, and there is no affordance left when scrolling cannot ask.

2. Arrow keys do nothing. Commit rows are <button role="row"> with no onKeyDown and no roving tabIndex. Measured: ArrowDown / ArrowUp move neither focus nor selection; Tab walks row by row, so a 40-row page is 40 tab stops and a virtualized 500-row history has no stable tab stop at all. The Sessions list (App.tsx moveSessionFocus) and ProjectSidebar already establish the convention this list is missing. aria-selected on role="row" is also only valid inside a grid, and the container is role="table".

3. The log pane holds dead height above the details pane. The workbench's first grid row is a fixed var(--git-log-height, 58%), so the log pane reserves 58% of the frame whatever it has to put there. A window-narrowed history of a dozen rows ends well short of the pane's bottom edge, and the reader sees a band of empty workspace between the last commit and Commit details.

Non-goals

• Pushing the date window into /api/git/log. Git range-limits on committer date (--since / --until) while the View filters and displays author date, so a server-side window would silently disagree with the column the reader sorts by. The window stays a client-side narrowing of loaded history.
• Selection following focus. Selecting a commit fetches /api/git/commits/\{sha\}; arrow keys must not fire a request per keystroke. Focus moves, Enter / Space selects.
• Redesigning the commit graph, the details pane, the refs tree, or the diff.
• Persisting pane sizes or the focused row across reloads.
• Reinstating the loaded-of-total footer line removed by docs/specs/2026-09-08-studio-commit-view-chrome.md AC-9.

Acceptance criteria

• AC-1: Entering the Commits View issues exactly one /api/git/log request, and changing the date window issues exactly one. No page is fetched without reader input, at any window setting.
• AC-2: When the loaded rows extend more than 160px past the viewport, scrolling to within 160px of the end fetches the next page, and each such arrival fetches at most one page. Reaching the end again fetches the next.
• AC-3: When the loaded rows do not extend more than 160px past the viewport and more history exists, the list ends with a Load older commits control that fetches the next page. It is a tab stop, names the remaining count, and is the only paging affordance in that state.
• AC-4: The paging footer keeps its AC-9 contract: empty while idle and complete, a live status while a page loads, Retry loading history after a page fails. The Load older commits control is absent in all three of those states.
• AC-5: The commit list is one tab stop. Exactly one row carries tabIndex=0; every other row carries -1. Tabbing into the list lands on the focused row, or on the first rendered row when the focused row has been virtualized away.
• AC-6: ArrowDown / ArrowUp move focus by one row, PageDown / PageUp by ten, Home / End to the first / last loaded row. Focus does not wrap, the virtualizer scrolls the target row into view before it takes focus, and no /api/git/commits request is made.
• AC-7: ArrowDown on the last loaded row requests the next page instead of moving focus, so a keyboard reader can travel past the loaded window.
• AC-8: Enter or Space on the focused row selects that commit and loads its details, leaving exactly one row with aria-selected="true".
• AC-9: The list container is role="grid", data cells are gridcell, and header cells are columnheader, so aria-selected on role="row" is valid.
• AC-10: When the log content is shorter than the log pane's share of the frame, the pane shrinks to its content and the details pane takes the freed height; no empty band remains between the last commit row and the details pane header. When the content is taller, the pane stays at its sash size.
• AC-11: No console or page errors, and no change to the narrow (stacked) layout's pane switching, at 1600×1000 and 390×844.

Plan

1. GitHistoryView.tsx — CommitTable:
   • Replace the sentinel and its IntersectionObserver with an onScroll prefetch that fires only while the scroll range exceeds PREFETCH_MARGIN, so one scroll to the end buys one page and a non-scrollable list buys none.
   • Track that scroll range with a ResizeObserver so the manual affordance and the auto path are decided by the same measurement.
   • Render a trailing git-load-older row when paging cannot be scrolled for.
   • Add focusIndex + roving tabIndex, an onKeyDown per row matching moveSessionFocus, scrollToIndex before focusing, and the last-row ArrowDown page request.
   • Move the container to role="grid", cells to gridcell, header cells to columnheader.
2. workbench.css: first workbench row becomes fit-content(var(--git-log-height, 58%)); drop .git-auto-load-sentinel; add .git-load-older.
3. i18n (en/git.ts, zh-CN/git.ts): add log.loadOlder with the remaining count and table.rowsAria; drop nothing.
4. test/browser/git-history.spec.mjs: cover AC-1 through AC-10.

Test and review evidence

• npm run typecheck and npx vitest run in packages/harness-studio: 80 files, 598 tests passing.
• npx playwright test test/browser/git-history.spec.mjs: both tests passing, including the new AC-1…AC-10 walk.
• npx playwright test (whole browser suite): 90 passed, 2 failed — project-shell.spec.mjs:193 and shared-scope.spec.mjs:70. Both reproduce identically in a clean b11a001 worktree with this change absent, so they belong to other in-flight Sessions/Inspector work in this shared checkout, not to this change.
• A live probe against a local Studio on the better-harness checkout (505 reachable commits): one /api/git/log on entry and none after a second of idling; one page per scroll to the end; ArrowDown/ArrowUp/PageDown/Home travelling by 1/1/10/first with a single tabIndex=0 throughout and no /api/git/commits request; Enter selecting one row and filling the details pane; Today adding no request and offering Load older commits · 426 left in history, whose count steps to 386 after one click; and a 6px log-to-details gap where the content fills the pane.
• Pane geometry at a 3-row log (Playwright) and a 4-row log (live, 1600×1000): log pane under 40% of the frame, details pane over 50%, refs pane still full height and still scrolling.

Risk

• A dropped page for a reader who never scrolls. AC-1 trades eager loading for reader-driven loading, so a window-narrowed history now shows only what the first page happened to contain until the reader asks for more. AC-3's control names the remaining count so the shortfall is visible rather than silent.
• Deviating from the Sessions list's wrap-around focus. Sessions wrap because the list is closed; the commit log is a paged timeline where wrapping from the oldest loaded commit to HEAD would misreport the history's shape. AC-6 and AC-7 clamp and page instead.
• A sash whose value can be clamped by content. Under AC-10 the log pane can be shorter than the sash's aria-valuenow, and dragging it taller has no effect while the content is short. The alternative is holding a void open, which DESIGN.md rejects; the sash still governs the pane whenever content fills it.
• fit-content() and the spanning refs pane. The refs pane spans all three rows, so an intrinsic first row could in principle be inflated by the refs tree. Measured against a 4-row log at 1600×1000: log pane 191px, details 641px, refs pane still the full 838px and still scrolling — the span does not inflate the track, so no containment hack is needed.