Window the top-level timeline on phones and evict far pages (C1 step 2)

[SawyerHood]

What was wrong

Every loaded timeline page stayed mounted. TimelineRowsList rendered one wrapper per top-level row, prependOlderTimelineRows concatenated pages without a cap, and auto-load pulled the next page 600px before the top. On a phone, reading back through a long thread left hundreds of markdown bodies and action bars in the DOM. Each style/layout pass (keyboard, orientation, streaming growth) and the memory footprint grew with how far the user had scrolled. Step 1 (content-visibility: auto, already on this branch) skips layout and paint of off-screen rows; the DOM itself was still unbounded.

What changed

• New timeline-windowing.ts (pure geometry): entry layout with measured heights or per-kind estimates plus the flex gap, binary search for the entry at an offset, range = viewport +/- overscan (2 viewports, >= 1600px), hysteresis (rangeCoversViewport with half-overscan slack), id-based range/anchor resolution that survives prepends, and segment building (mounted entries + spacers sized to the rows they replace).
• New useTimelineWindow.ts: samples the bottom-anchored scroll body on rAF-throttled scroll and on scroll-area resize; measures mounted wrappers with one ResizeObserver; keeps a viewport anchor by row id (null = bottom); re-ranges only when the viewport nears an edge; corrects scrollTop in the same layout pass by keeping the first visible mounted row in place (idempotent under Chromium scroll anchoring, does the job on WebKit); snaps the AutoHeightContainer on a swap; pins the live turn, unread divider, search-target ancestors and the focused row; skips swaps while a text selection is inside the list; seeds the first window from the saved per-thread scroll anchor; renders everything until sampled when windowing switches on later; re-anchors from the DOM when the anchored row is replaced.
• ThreadTimelineRows.tsx: the top-level list renders windowed segments (spacers carry data-timeline-row-spacer, overflow-anchor: none) when compact and inside a scroll body; nested lists, desktop and surfaces without a scroll body render exactly as before. Unread divider gets a measured wrapper. Manual expand/collapse choices are remembered per row for the timeline instance so evicted rows re-mount in the state the user left them.
• ExpandableTimelineRow: optional initialManualExpansionOverride / onManualExpansionOverrideChange seed props (state stays local).
• height-transition.tsx: useAutoHeightSnap() context so the window can snap the wrapper after a swap.
• timeline-row-containment.ts: estimateSkippedTimelineRowBlockSizePx shared by containment and windowing so spacer and skipped rows agree.

How you verified

• New timeline-windowing.test.ts (13 tests): layout with gaps and measured heights, offset lookup, range with overscan and clamping, coverage/hysteresis, anchor resolution across a prepend, id round-trips, spacer heights add up to the un-windowed list height.
• New ThreadTimelineRows.windowing.test.tsx (14 tests, jsdom with a simulated flex layout, ResizeObserver and rAF): all rows render when not compact or without a scroll body; initial window is the trailing block plus one estimated spacer; scrolling to the top mounts the head, evicts the middle and keeps the last row; no re-render inside the slack; measured heights size the spacer and rows re-mount in place; scrollTop is corrected when re-mounted rows are taller than their spacer share; a prepended page keeps the same rows mounted and grows the spacer by exactly the page's estimate; the anchored row being replaced keeps the window and re-anchors instead of jumping; running turn rows stay mounted far below; the search target's top-level row stays mounted; the saved scroll anchor seeds the first window; windowing that switches on after mount does not evict the current view; the unread divider is wrapped and pinned; a manually expanded row is still expanded after eviction and re-mount.
• pnpm exec turbo run typecheck --filter=@bb/app: pass. pnpm exec turbo run lint --filter=@bb/app: 0 errors (6 new react-hooks/refs warnings in the hook for the deliberate controller reads, same category as existing timeline code). Vitest over src/components/thread, src/components/ui, src/components/plugin, src/views: 126 files / 1086 tests pass, including every existing timeline, scroll-body and containment test.

Fixes: part of the mobile / iOS Safari performance program (verified sweep report in the bb thread; no single issue).

Stack context

Layer 26 of 28 in the bb/mobile-perf/* stack (bottom → top: quick wins first, big rocks last).

• Prerequisite (layer below): bb/mobile-perf/composer-draft-pushdown (Push the composer draft subscription below ThreadDetailPromptArea (D1) #1904).
• Next layer: bb/mobile-perf/persisted-query-cache (Persist an allowlisted query cache slice to IndexedDB behind an experiment (J2) #1906).
• Audit findings addressed: see title IDs. Review: approved-with-fixes; 1 review fix commit(s).
• Reviewer notes / follow-ups: Not fixed (timing-dependent, self-healing): when a thread with a saved non-bottom scroll anchor mounts with rows arriving in a later commit than the scroll body (cache miss after g | Pre-existing/inherited from C1 step 1, noted by the implementer: rows re-mounted above the viewport carry the content-visibility estimate until realized, so WebKit (no scroll ancho | Design trade-off, not a bug: on compact viewports AutoHeightContainer is snapped on every window swap, so the timeline height glide is lost on phones (aligned with C2 direction).
• Wire/contract: None. No server/daemon contract, WebSocket message, host RPC, route, CLI flag or config knob changed; HOST_DAEMON_PROTOCOL_VERSION untouched. Only apps/app client code.

AGENT GENERATED: by Claude Opus 5

[SawyerHood]

Closing: the maintainer decided to land only layers 1–22 of the mobile-perf stack (#1880–#1901). The branch stays on the remote for reference; it can be reopened and rebased later if the remaining big rocks (sidebar-bootstrap patching, thread bootstrap include=, composer draft push-down, timeline windowing, persisted cache, service worker) are picked up again.