Pause live output follow while reading

[lnieuwenhuis]

Summary

• stop following streamed output once the reader scrolls away from the live edge
• preserve the current reading position while new output arrives
• resume following when the reader returns to the bottom or uses the existing scroll-to-end control

Why

LegendList's internal end-maintenance remained enabled after manual navigation, so streamed layout and data updates could pull the viewport back down even after the timeline scroll mode had detached.

Closes #3925

Validation

• vp test apps/web/src/components/chat/MessagesTimeline.test.tsx
• vp check
• vp run typecheck

Note

Pause live output auto-follow in chat timeline during manual scroll navigation

• Adds manual navigation detection to MessagesTimeline via wheel, touch, keyboard, pointer, and minimap interactions; confirmed upward navigation disables auto-follow and shows the scroll-to-bottom pill.
• Auto-follow resumes only when the user scrolls back to the bottom after confirmed manual navigation, using the new shouldResumeTimelineAutoFollow logic in timelineScrollAnchoring.ts.
• Auto-follow state is tracked per-route and resets to enabled on route change via resolveTimelineAutoFollowEnabledForRoute.
• LegendList maintainScrollAtEnd is disabled when auto-follow is off or a navigation probe is active, preventing the list from snapping back to the bottom during reading.
• Behavioral Change: onIsAtEndChange now receives a structured { isAtEnd, manualNavigationReachedEnd } object instead of a plain boolean.

^{Macroscope summarized 7be9742.}

---

Note

Medium Risk
Scroll and follow behavior is intricate and user-facing; the onIsAtEndChange callback shape changed, though coverage is added in unit tests.

Overview
Stops the chat timeline from pulling the viewport back to the live edge while the user reads older messages during streaming. LegendList maintainScrollAtEnd is now tied to an explicit autoFollowEnabled flag instead of staying on after the parent thought follow was off.

MessagesTimeline detects real navigation toward history (wheel, touch, keyboard, scrollbar drag, minimap jumps) via capture handlers and a short navigation probe, then calls onManualNavigation. Scroll handling reports { isAtEnd, manualNavigationReachedEnd } so ChatView only re-enables follow when shouldResumeTimelineAutoFollow agrees (e.g. user scrolled back down after detaching, not spurious “at end” during new-turn anchoring). Thread/route switches reset follow on by default through resolveTimelineAutoFollowEnabledForRoute.

The old ChatView scroll-node listeners on wheel/touch/pointerdown are removed in favor of this timeline-owned logic. resolveTimelineIsAtEnd now prefers isWithinMaintainScrollAtEndThreshold so “near end” maintenance does not count as the true live edge.

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR introduces a new user-facing feature that pauses live output auto-follow when the user scrolls up to read history. It adds substantial new logic for detecting user navigation intent across multiple input types (wheel, touch, keyboard, scrollbar) and modifies the timeline's auto-scroll behavior. New features with runtime behavior changes warrant human review.

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[lnieuwenhuis]

Sorry for closing without warning, I thought this PR was stale and only saw @juliusmarminge self-requested a review an hour ago after I closed it. Hope this didn't fuck anyone's workflows.

[lnieuwenhuis]

@juliusmarminge I think it's ready if you wanna merge it

[cursor]

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

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