fix(web): stabilize chat timeline positioning

[jmeistrich]

What Changed

Fixed some usage of LegendList to make the chat smoother. This mostly removes app code in favor of using LegendList's internal behavior.

Why

Chat was not as smooth as it could be.

UI Changes

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

---

Note

Medium Risk
Large removal of scroll state in a core chat UI with behavior changes during streaming, send anchoring, and expand/collapse; risk is regressions in scroll position rather than data/security.

Overview
Bumps @legendapp/list from 3.2.0 to 3.3.3 and shifts chat timeline scroll behavior onto the list’s built-in maintainScrollAtEnd / maintainVisibleContentPosition instead of custom logic.

Removes the manual scroll system in ChatView (scroll modes, live-follow refs, wheel/touch/pointer opt-out, rAF follow loops, anchor size restore) and deletes timelineScrollAnchoring.ts and its tests. Send/scroll-to-bottom still set timelineAnchor and call scrollToIndex / scrollToEnd, but no longer drive anchoring metrics or manual navigation cancellation.

In MessagesTimeline, maintainScrollAtEnd stays on (non-animated, on data/layout changes) even when anchoredEndSpace is active—previously it was disabled during anchoring. Fold/expand for turns and work groups briefly turns off end maintenance and uses shouldRestorePosition scoped to the toggled row; the old flushSync + DOM scroll delta compensation on work-group toggles is gone. onAnchorSizeChanged and onManualNavigation are dropped from the timeline API.

^{Reviewed by Cursor Bugbot for commit 8cf8708. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Stabilize chat timeline scroll positioning by replacing manual anchor logic with maintainScrollAtEnd

• Removes the custom TimelineScrollMode anchoring system and all associated refs, state, and scroll-restore logic from ChatView.tsx and MessagesTimeline.tsx, deleting timelineScrollAnchoring.ts entirely.
• Introduces a TIMELINE_MAINTAIN_SCROLL_AT_END config in MessagesTimeline using @legendapp/list v3.3.3's maintainScrollAtEnd (bumped from 3.2.0), with animated=false and all layout change hooks enabled.
• When a workgroup row is folded/unfolded, maintainScrollAtEnd is briefly suspended via suspendEndScrollMaintenanceForDisclosure to let the layout settle, with maintainVisibleContentPosition scoped to the toggled row via shouldRestorePosition.
• WorkGroupToggleTimelineRow no longer performs manual DOM measurement or scroll delta compensation on click — position maintenance is fully delegated to the list.
• Behavioral Change: onAnchorSizeChanged and onManualNavigation props are removed from MessagesTimeline's public API.

^{Macroscope summarized 8cf8708.}

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[chatgpt-codex-connector]

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Reviewed commit: 221222ccb8

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[chatgpt-codex-connector]

Restore end-follow while an anchor remains

When a sent message remains as anchorMessageId, MessagesTimeline still passes maintainScrollAtEnd={false} whenever anchoredEndSpace is truthy, so this effect is the only fallback that keeps an at-end timeline pinned during later streaming/final timeline updates. If onIsAtEndChange flips timelineScrollModeRef back to following-end while that anchor is still present, this new guard skips the old scrollToEnd path and subsequent rows/size changes can stop following even though the user never manually navigated away.

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[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR refactors the chat timeline scroll anchoring system, removing ~600 lines of custom logic in favor of library features. This constitutes a significant behavioral change to a core UX element. An unresolved review comment also flags a potential bug where scroll-following may stop working when an anchor is present.

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