fix(web): find shows its match after a thread switch, and the timeline hears scrolling through one viewport signal#25462

Problem

The chat timeline's scroll input (wheel, touch, scrollbar drag, scroll keys, scroll that chains out of an embedded frame) was heard separately by the follow hook, the position restore and MessagesTimeline. Each had its own listener effect, and a "manual navigation" callback plus a cancelPositionRestoreRef were threaded through ChatView and GuestPage. One visible result: when the list switched to a thread that already had find open, the position restore and find's reveal both scrolled the list, and the restore won. You saw the remembered row, not the match.

Change

• New timelineViewport.ts. The list's ref callback attaches it to the scroll node. It owns the input listeners and emits one TimelineNavigation signal, and it also holds the follow state machine (transitionTimelineFollow, navigationReleasesFollow) as pure functions.
• useTimelineFollow subscribes through onViewportAttach (replacing onManualNavigation and cancelPositionRestoreRef). Follow mode lives in a small store read with useSyncExternalStore, keyed per thread visit, so a thread switch needs no reset effect.
• useTimelinePositionRestore becomes startTimelinePositionRestore, a plain function called from one layout effect. It stops when the viewport signals navigation.
• Find reveals its match only after the opening restore settles.
• Opening find now releases follow from the find store's open action (announceOpening, a navigate field on the attached find timeline), instead of an effect on findOpen.
• MessagesTimeline re-measures content size, overflow and the remembered position from LegendList's totalSize listener and the lane's resize observer, instead of effects keyed on rows.length and chatWidth. The interrupted-run fold is now opened and closed during render.

What this improves

• Bug fixed: "reveals find's match, not the remembered row, on a thread it switches to" (MessagesTimeline.browser.test.tsx). It fails on main: the match row ends up 4500px above the viewport.
• Sync removed: three separate input-listener effects become one viewport attachment. The findOpen -> navigation effect, the follow and restore reset effects, the settle double-rAF effect, and the rows.length/chatWidth re-measure effects are all gone.
• New tests: timelineViewport.test.ts covers the follow transitions and the list-opening logic. threadFindStore.test.ts checks that opening find navigates once and reopening does not.

Behavior changes

• Opening find on the thread already shown now releases follow from the open action, not from a render that saw findOpen flip. A list that starts showing a thread whose find is already open releases follow once.
• After a thread switch, the "settling" flag clears when the new thread's first content is laid out, not after two animation frames.
• When follow is released, the remembered position is rewritten as "not following" right away, without waiting for a scroll event.
• On a chat-width change, the minimap gutter is re-measured by the content lane's resize observer, not by an effect on the setting.
• Disclosure settle frames are no longer cancelled on unmount; they only clear a flag.
• No logging changes.

Effects

MessagesTimeline.tsx 10 -> 1, useTimelineFollow.ts 6 -> 1, useTimelinePositionRestore.ts 2 -> 0. ChatView.tsx 20 -> 20, GuestPage.tsx 7 -> 7, useThreadFindPositionReader.ts 1 -> 1. Two effects are kept on purpose:

• the layout-effect position restore, which has to land before paint and after LegendList's own layout effects apply the new data;
• the anchor-release scroll, which has to follow the commit that drops the anchored end space.

Verification

• vp test run timelineViewport.test.ts threadFindStore.test.ts useTimelinePositionRestore.test.ts MessagesTimeline.browser.test.tsx: 4 files, 43 tests pass.
• I swapped in main's MessagesTimeline.tsx, useTimelinePositionRestore.ts, threadFindStore.ts and useThreadFindPositionReader.ts: "reveals find's match…" fails (expected -4500 to be greater than or equal to 0).
• vp run check:changed: format, lint, knip, typecheck and tests (10 files, 233 tests) all pass.

Model: Claude Opus 5.5, Claude Code harness.

🤖 Generated with Claude Code