perf(tabs): scroll the active tab into view once per activation, not three times

[h4yfans]

Closes #1105. Part of epic #987 (memory/CPU audit 2026-08).

What was wrong

apps/desktop/src/renderer/src/components/tabs/tab-bar-with-drag.tsx:111-116 (pre-fix) — one tab activation fired three smooth scrollIntoView animations:

useLayoutEffect(() => {
  if (!activeTabId || activeDragItem) return
  const tabEl = scrollRef.current?.querySelector(`[data-tab-id="${CSS.escape(activeTabId)}"]`)
  tabEl?.scrollIntoView?.({ inline: 'nearest', block: 'nearest', behavior: 'smooth' })
}, [activeTabId, regularTabsLength, activeDragItem, canScrollToStart, canScrollToEnd])

canScrollToStart / canScrollToEnd are gutter flags, and each one that flips adds a ps-7 / pe-7 gutter to the strip. The activation scroll is what makes them flip, so the effect re-entered twice more and re-issued a smooth scroll each time, restarting the animation mid-flight. Measured on a strip that overflows: 3 scrollIntoView calls for a single activation (1 on activation, +1 when canScrollToEnd flips in the same commit, +1 when canScrollToStart flips on the first scroll event).

The gutter re-runs are not pointless — a widening gutter really can push the tab back out of view — so they could not simply be dropped from the deps.

What changed

• The effect now remembers which tab it last scrolled to (scrolledTabIdRef). A run whose target changed always animates; a repeat run for the same tab (i.e. a gutter-driven one) re-scrolls only if the tab is no longer fully inside the strip. isTabFullyVisible compares the rects and subtracts the strip's inline padding, so a tab hidden under a chevron still counts as out of view — the correction the old deps existed for is preserved, minus the redundant animations.
• Docs: apps/docs/src/user-guide/tabs-split-view.md — one line on the single scroll animation, one line stating tabs are never auto-closed.

Deliberately NOT done: the tab cap

The issue also asks for a maxTabs soft cap with LRU-close. I did not add one, on purpose:

• Every cap variant can destroy user state. LRU-closing loses a tab the user deliberately kept (and recentlyClosed only retains 25 entries, in memory, so it is not a safety net across a restart). Refusing to open past a limit means clicking a note in the sidebar silently does nothing. Both are worse than the thing they fix. This is a live-user build; tab state is persisted.
• The cost being capped is already small and already bounded. Hidden tabs are not mounted — split-view/tab-pane.tsx:57 renders <TabContent> for the active tab only — so an idle tab costs one record plus its viewState. The other per-tab accumulators are already capped: activation history at 50 (reducers/history-helpers.ts:3), recently-closed at 25 (reducers/tab-crud-reducer.ts:29), and the persistence serialize + JSON.stringify now runs once per debounce window rather than per state change (perf(tabs): serialize the tab tree once per debounce window #1174, which closed [HIGH][tabs] Tab persistence serializes + JSON.stringifies the whole tab tree on every state change (debounce only covers the write) #1056 — re-verified in persistence/hooks.ts:62-88).
• The user-facing remedy already exists and is non-destructive: Close others / Close to the right / Close all in the tab context menu, now called out in the docs.

The only maxTabs reference in the tree is a stray field in a test fixture (persistence/persistence.test.tsx:112) that is cast away and read by nothing; left untouched as pre-existing dead data. Since no cap is introduced, a persisted session of any size still restores in full.

How it was proven

New apps/desktop/src/renderer/src/components/tabs/tab-bar-scroll.test.tsx drives the real component with stubbed strip geometry (scrollWidth / clientWidth / scrollLeft), so the gutter flags flip exactly as they do in the app, and counts scrollIntoView calls.

	after mount+activation	after the second gutter flips	after switching to another tab
before (origin/main source, same test)	2	3	3
after	1	1	2 (1 per activation)

• Before: git checkout origin/main -- …/tab-bar-with-drag.tsx → Test Files 1 failed (1) / Tests 2 failed (2) — expected "vi.fn()" to be called 1 times, but got 2 times.
• After: Test Files 1 passed (1) / Tests 2 passed (2).

Verification

./node_modules/.bin/vitest run --config apps/desktop/config/vitest.config.ts --project renderer \
  src/renderer/src/components/tabs src/renderer/src/components/cold-major-components.test.tsx \
  src/renderer/src/components/small-zero-surfaces.test.tsx src/renderer/src/components/split-view
  → Test Files 17 passed (17), Tests 90 passed (90)
    (includes the pre-existing "scrolls the active tab into view so it never stays
     past the strip edge" case, still green)

pnpm --filter @memry/desktop typecheck:web   → clean
pnpm exec eslint …/tab-bar-with-drag.tsx     → 0 errors
git diff --check                             → clean
pnpm docs:impact --base origin/main --strict → covered
pnpm docs:build                              → build complete

Backward compatibility

Render-time only: no schema, contract, settings, persisted-tab-state, or sync-shape change, and no cap that could drop tabs from an existing session.

[github-actions]

React Doctor found no new issues. 🎉

_{Reviewed by React Doctor for commit a463d1b.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!