Let taps paint: transition-priority navigation, deferred expanders and sidebar realization

[vburojevic]

What was wrong

Interaction taps on mobile web ran their entire React consequence as one discrete synchronous commit, so the browser could not paint tap feedback first. The perf-watch ledger attributes 671s/267 stalls over 7 days to click on button.inline-flex (the ExpandablePanel header and shared Button class) and 858s/364 stalls to after navigate: the expander body materialized inside the click's render, openMobileSidebar flushed the whole ProjectList/ThreadRow subtree before the slide's first frame, navigation exposed no pending signal, and the split-pane maximize toggle rewrote scrollLeft/scrollTop on every tracked element for 30 unconditional rAF frames.

What changed

• app-route-anchor.tsx: navigateRoute wraps navigation in useTransition's startTransition; the navigate context identity is unchanged, and isPending is exposed through a separate RouteNavigationPendingContext + useIsRouteNavigationPending() so navigate consumers don't re-render per navigation. RootComposeMobileRecents.tsx swaps its raw <Link> for RouteAnchor. (react-router v7 already transitions router state; the delta is the pending signal plus the remaining urgent work riding the same transition.)
• disclosure.tsx: the expander body realizes off useDeferredValue(isExpanded) — caret/header stay urgent, the body mounts in a follow-up interruptible commit; closing-body retention and the layout-animation signal unchanged.
• sidebar.tsx: realizeMobileSidebar() runs inside React.startTransition; drag-style write order unchanged; the settle commit's render-phase latch still covers the race. The file's flushSync sites were reviewed individually and deliberately left alone.
• SplitThreadArea.tsx: restore() compares before writing and returns whether anything moved; the loop stops after the first zero-correction frame, capped at 5 frames.

How you verified

• New tests, each proven fail-before/pass-after: transition navigation samples {isPending, old pathname} in the tap's commit; the deferred body is absent from the urgent flush and lands in the deferred commit (plus a closing-body retention guard); the sidebar realize commit is absent from the tap's flush; settled restore performs 0 scroll writes (before: 31), an adversarial scroller is corrected ≤6 times.
• pnpm exec turbo run typecheck --filter=@bb/app exit 0; focused suites 67 files / 620 tests pass; lint 0 errors.
• Merged-tree WebKit QA (iPhone viewport): section expand, sidebar open/close, thread navigation — all pass with zero console errors.

AGENT GENERATED

[SawyerHood]

Closing in favor of #2435, which integrates this PR on current main together with #2385, #2386, #2388, #2389 and #2392 and the fixes from the adversarial review. Your commits are cherry-picked unchanged with you as author, and every fix commit carries a Co-authored-by trailer for you, so credit follows the work whichever merge method is used. The branch stays as-is.

Fix landed there for this PR:

• In disclosure.tsx the region's content branch, height sync, transition classes, deadline and in-flight accounting now key on the deferred expanded value, so the collapsed preview stays until the body exists and the tween starts from the real body; re-expanding inside the 200 ms close window keeps the retained body.
• Left as-is: the transition-priority navigation is a no-op because react-router already transitions; the deferred-body starvation under heavy streaming is an open trade-off noted in Mobile perf: integrate #2385, #2386, #2388, #2389, #2392 with the review fixes #2435.

Thanks for the work: the design held up under review; only the items above needed changing.

AGENT GENERATED