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 follow in chat timeline while user navigates toward history

• Introduces autoFollowEnabled state in ChatViewContent to control whether the message timeline auto-follows the live end; this replaces direct wheel/touch/pointer listener attachment on the scroll node.
• MessagesTimeline gains an autoFollowEnabled prop and new capture-phase event handlers (wheel, touch, keyboard, pointer) that detect upward/history-directed input and mark a manual navigation session, disabling maintainScrollAtEnd on LegendList.
• Auto-follow is re-enabled when the user scrolls back to the end, sends a message, switches threads, or calls scroll-to-end; it is disabled when the user navigates away from the end.
• New utilities in MessagesTimeline.logic.ts standardize directional input detection, scroll-node end resolution, and manual navigation session tracking.
• Behavioral Change: resolveTimelineIsAtEnd now gives highest precedence to isWithinMaintainScrollAtEndThreshold, changing how the live-edge is determined when that value is present.

^{Macroscope summarized 0e32afa.}

---

Note

Medium Risk
Scroll and live-follow behavior is easy to regress across wheel, touch, keyboard, and streaming layout updates, though logic is extracted and covered by new unit tests.

Overview
Stops the chat timeline from snapping back to the live edge while the user reads older messages during streaming.

ChatView now tracks timelineAutoFollowEnabled and passes it to the timeline. Auto-follow turns off when the user navigates away from the bottom (via onManualNavigation) and turns back on when they reach the end, send a message, scroll to end, or switch threads. The old useEffect that listened for wheel/touch/pointer on the LegendList scroll node is removed.

MessagesTimeline wires capture-phase handlers (wheel, touch, keyboard, scrollbar pointer) that only treat input as “read history” when it moves toward older content and the list can actually scroll up. That calls onManualNavigation and turns off LegendList maintainScrollAtEnd when autoFollowEnabled is false (also briefly during keyboard-up navigation). Scroll handling uses new logic helpers to decide “at end” vs returning to the live edge after manual scroll.

MessagesTimeline.logic adds pure helpers for directional input, scroll overflow, DOM-based end detection, and prefers isWithinMaintainScrollAtEndThreshold over isNearEnd for live-edge visibility.

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR introduces significant changes to how auto-scroll/auto-follow works in the chat timeline, including new event handlers to detect user navigation and pause auto-follow while reading. This represents a meaningful user-facing behavior change that warrants human review.

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

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