fix(web): reopen a thread that was following its stream at the end

[santiago-ramos-02]

Fixes #13601

What Changed

MessagesTimeline now remembers a thread as at the end while live follow is active (atEnd: isAtEnd || (liveFollowEnabled && isLiveFollowLatched() && !anchoredEndSpace)). Once the user scrolls, uses the minimap, or otherwise navigates away, ChatView releases its follow latch and the real offset is saved as before. isLiveFollowLatched reads that latch directly, because a gesture releases it before liveFollowEnabled re-renders. A thread's first send, held near the top by anchored end space, is not following the end, so its position is kept too.

A few lines in MessagesTimeline.tsx and ChatView.tsx, plus a test that fires a scroll with the list 500 px short of the end. With follow on, the thread is remembered at the end. With the latch released, follow off, or the first send anchored, the same offset is kept as a reading position.

Why

While a turn streams, maintainScrollAtEnd glides to the end, so new output sits below the viewport until the follow scroll catches up. Scroll events in that window report "not at end", and handleScroll saved that into the per-thread position cache. Switching away at one of those moments stored a following thread as a reading position. Switching back after the reply finished then restored that offset, often thousands of pixels above the latest message, with "Scroll to end" showing.

ChatView already treats this gap as follow lag: onIsAtEndChange ignores isAtEnd === false while live follow is active. The remembered position now uses the same rule, so the restore path and ChatView's thread-switch effect agree that the thread was following.

I confirmed the cause by logging every rememberTimelinePosition call in a dev build. One streamed reply produced 68 atEnd: false writes while nothing scrolled the timeline by hand. I also checked these cases in the browser:

• Following, switch away mid-stream, return after it finishes: opens at the end.
• Scroll up with PageUp mid-stream, switch away, return: the reading position is restored with the pill, unchanged from before.
• Switch to a thread not yet opened in the session, which briefly paints the previous timeline: no stale write.

This does not touch the separate cases in #12372 (pill hidden after late row measurement) or #12222 (restore against stale rows).

UI Changes

Before: the thread follows the stream, I switch threads mid-reply and come back after it finishes. It opens 2,657 px above the end.

timeline-before-fix.mp4

After: the same steps, switching at a moment when the follow scroll was behind. It opens at the end.

timeline-after-fix.mp4

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

Tested with vp test run on MessagesTimeline.test.tsx and timelineScrollAnchoring.test.tsx, plus lint, format and typecheck for apps/web.

Model: Claude Opus 5.5 (1M context), running in Claude Code inside T3 Code.

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Summary by CodeRabbit

• Bug Fixes
  • Saved chat positions now reflect whether live-follow is still latched. When it is active and there’s no anchored end space, positions are marked at the end; after a user gesture releases the latch, the actual scroll position is preserved.

[macroscopeapp]

Approvability

Verdict: Approved at 83a7ea4

Macroscope's review found this PR approvable — This is a small, localized web bug fix that reconciles remembered thread positions with the existing live-follow state during streaming. Production changes are limited to scroll-position persistence, with targeted regression coverage and no default, schema, infrastructure, security, billing, or static-analysis changes.

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📒 Files selected for processing (3)

• apps/web/src/components/ChatView.tsx
• apps/web/src/components/chat/MessagesTimeline.test.tsx
• apps/web/src/components/chat/MessagesTimeline.tsx

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• apps/web/src/components/chat/MessagesTimeline.tsx
• apps/web/src/components/chat/MessagesTimeline.test.tsx

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📝 Walkthrough

Walkthrough

Saved timeline positions now account for whether live follow remains latched. ChatView provides the latch state to MessagesTimeline, which uses it when recording whether a saved position is at the end. Tests cover latched and unlatched states.

Changes

Live-follow position persistence

Layer / File(s)	Summary
Live-follow saved position apps/web/src/components/ChatView.tsx, apps/web/src/components/chat/MessagesTimeline.tsx, apps/web/src/components/chat/MessagesTimeline.test.tsx	ChatView passes the live-follow latch state to MessagesTimeline. The scroll handler records atEnd: true when the timeline is at the end, or when live follow is enabled, the latch is active, and no anchored end space exists. Tests check saved positions with the latch active and released, disabled live follow, and a first-send anchor.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: juliusmarminge

Merge Risk: ⚪ Minimal · up to 83a7e

When live follow is released, the timeline saves the actual reading position. No actionable merge risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes satisfy issue [#13601]. MessagesTimeline records atEnd: true when live follow is enabled, the live-follow latch is active, and no anchoredEndSpace exists. This prevents follow-lag sc…
Out of Scope Changes check	✅ Passed	The pull request changes MessagesTimeline, ChatView, and the focused MessagesTimeline test. The production changes provide the live-follow latch required by issue [#13601]. The test verifies the…
Title check	✅ Passed	The title clearly describes the main change: reopening a thread at the end when it was following its stream.
Description check	✅ Passed	The description is complete and follows the repository template. It explains what changed, why, UI behavior, validation, and includes UI attachments and checklist updates.

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[santiago-ramos-02]

Thanks for the triage on #13601, @juliusmarminge. On your two review notes:

Anchored first send: agreed, fixed in b61dd88. A gap under anchored end space is a real position, so the OR now applies only when there is no anchored end space: isAtEnd || (liveFollowEnabled && !anchoredEndSpace). The test covers it. Without the guard, that case is remembered as at the end.

State lagging the latch ref: the window is one frame. This PR adds liveFollowEnabled to handleScroll's dependencies, so when follow turns off, the existing [handleScroll, rows.length] effect runs handleScroll on the next frame and overwrites the stale atEnd: true with the real offset, even if no further scroll event arrives. A thread switch would have to land between the wheel or key event and the next frame. Closing that fully would mean passing the latch ref or a getter from ChatView into the timeline. I left that out to keep this small, but I'm happy to add it if you prefer.

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Actionable comments posted: 1

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Inline comments:
In `@apps/web/src/components/chat/MessagesTimeline.tsx`:
- Line 1036: In MessagesTimeline, synchronously save the current scroll offset
during manual navigation before disabling live follow, so a pending handleScroll
event cannot overwrite the saved reading position with an at-end state before a
thread switch.

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[santiago-ramos-02]

Update: CodeRabbit flagged the same race, so I closed it in 83a7ea4 instead of leaving it. ChatView passes its live-follow latch to the timeline as isLiveFollowLatched, and the save checks it along with liveFollowEnabled. A gesture that releases the latch now saves the real offset right away. The test covers the latch being released while liveFollowEnabled is still true.

Dismissing prior approval to re-evaluate 83a7ea4

[juliusmarminge]

Thanks for working on this. We merged the orchestrator V2 rewrite in #2829, and we are closing this PR as part of that transition.

The patch conflicts with the rewrite in apps/web/src/components/ChatView.tsx, apps/web/src/components/chat/MessagesTimeline.tsx. Even where the conflict is small enough to rebase, we are asking for fresh PRs against the new base so we can review and verify the behavior in V2.

Sorry for the extra work this creates. If the change is still needed on V2, please rebuild it on current main, verify it there, and open a new PR linking back here. We're closing the current implementation without assuming the underlying request is resolved.

[santiago-ramos-02]

No problem, thanks for the heads-up. I rebuilt this on current main with the V2 orchestrator and verified it there: #14916. On V2 it happens less often (plain streamed text keeps up now), but replies with tool calls still save a following thread as a reading position, so the fix still applies. Details and before/after videos are in the new PR.