fix(web): scroll-to-end pill no longer jitters after scrolling up by Melv1C · Pull Request #26365 · pingdotgg/t3code

Melv1C · 2026-10-28T08:32:59Z

After you scroll up in a long thread, the "Scroll to end" pill jumps up and down by tens of pixels, even while the thread is idle.

Summary

The pill sits above the composer, so it moves whenever the composer's height changes. Scrolling up collapses the composer into its resting layout, which is only allowed while the timeline overflows. The timeline's list ref callback (setTimelineList) was a plain closure that React Compiler left un-memoized, so it got a new identity on every render. React then detached and re-attached the list each time, and the detach reports "no overflow":

render ConversationTimeline
  new setTimelineList identity
    old ref cleanup → onContentOverflowChange(false)
      composer leaves resting layout (overlay 110px → 147.5px)
        pill moves up
    re-attach → next frame measures overflow → true
      composer rests again (147.5px → 110px)
        pill moves down
  → re-render → repeat (about every 25ms)

The fix makes the ref callback stable, so the list attaches once per instance. The old re-attach also re-ran the content-size check after every render: it re-measured overflow when the composer inset changed, and it ended thread settling after a switch. A layout effect now keeps that check without detaching:

-const setTimelineList = (list) => {
+const setTimelineList = useStableCallback((list) => {
   …
+  scheduleContentSizeChangedRef.current = scheduleContentSizeChanged;
   scheduleContentSizeChanged();
   return () => {
+    scheduleContentSizeChangedRef.current = null;
     …
     onContentOverflowChange?.(false);
   };
-};
+});
+useLayoutEffect(() => {
+  scheduleContentSizeChangedRef.current?.();
+});

Evidence

Reproduction. Dev server with a long Replay thread: scroll up with the wheel, then sample the composer overlay height and the pill's bottom every 8ms. The reporter's DevTools log on the desktop Nightly showed the same flip: overlayH 147.5 ↔ 110, pill bottom 152px ↔ 82px.

Before (four short wheel scrolls over 8s): the composer flips between 110px and 147.5px about every 25ms the whole time, including between scrolls. The pill jumps between 82, 114, 152 and 160px.

pill-before.mp4

After (same script): the composer collapses once when the scroll starts. The pill stays at bottom: 114px for the rest of the run.

pill-after.mp4

Test. MessagesTimeline.browser.test.tsx, "keeps reporting overflow while a parent re-renders it":

render an overflowing timeline → wait until it reports overflow = true
re-render it the way a parent update does
expect overflow never reported as false

It fails on main (onContentOverflowChange called with false) and passes with the fix. All 31 tests in the file pass, and vp run check:changed passes.

Not checked: I didn't run a packaged desktop build or check Linux. The issue's original Linux report (horizontal jumps) is most likely the same loop, but I haven't confirmed it.

Merge Danger

Door: two-way

Blast Radius: timeline

The list now attaches once instead of on every render. Thread switches, position restore, find, and follow-on-new-turn all hang off that attach. The existing MessagesTimeline browser tests for those paths pass.

Claude Code (Claude Opus 5.5) made this change. Claude Haiku, Claude Sonnet, and Cursor Grok 4.7 reviewed it.

🤖 Generated with Claude Code

The timeline's list ref callback got a new identity on every render, so React detached and re-attached the list each time. Detaching reports "no overflow", which pulled the composer out of its resting layout; the next measurement put it back, and the pill, positioned from the composer's height, jumped with every flip. The ref callback is now stable, so the list attaches once per instance. A layout effect keeps the per-render content-size check the re-attach used to run, so overflow and thread settling still re-measure. Fixes pingdotgg#26338 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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macroscopeapp · 2026-10-28T08:34:53Z

Approvability

Verdict: Needs Human Approval

Macroscope's review found this PR requires human approval — The change stabilizes timeline attachment to prevent scroll-to-end pill jitter while preserving overflow measurements. It also adds an oxlint suppression at MessagesTimeline.tsx:1198, which requires human review.

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coderabbitai · 2026-10-28T08:42:26Z

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

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

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Walkthrough

MessagesTimeline now schedules content-size handling after renders, including when content does not resize. A browser regression test checks that an overflowing timeline does not report false after a parent re-render with the same messages.

Changes: Timeline overflow updates

Layer / File(s)	Summary
Schedule and verify content-size updates `apps/web/src/components/chat/MessagesTimeline.tsx`, `apps/web/src/components/chat/MessagesTimeline.browser.test.tsx`	`setTimelineList` has a stable callback identity and stores the content-size scheduler in a ref. A layout effect invokes the scheduler after renders, and cleanup clears the ref when the list detaches. The browser test checks overflow state after a parent re-render.

Priority: Low

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

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: juliusmarminge, t3dotgg

Merge Risk: Minimal · previous review 15b07

The overflow regression is covered by the updated test, and no actionable issue remains before merge.

Pre-merge checks: 3 · 1

Failed checks: 1 inconclusive

Check name	Status	Explanation	Resolution
Linked Issues check		Issue [`#26338`] requires the “Scroll to end” pill to stay fixed while visible. The change stabilizes `setTimelineList` and rechecks overflow in `useLayoutEffect`. The regression test proves that a pare…	Provide reviewable evidence that the pill keeps a fixed horizontal position for the issue scenario, or add a regression test that verifies this behavior.

Passed checks: 3 passed

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main fix: preventing jitter in the web scroll-to-end pill.
Description check		The description covers the problem, root cause, fix, linked issue, reproduction evidence, focused test results, verification limits, and blast radius. The scope section does not explicitly state maint…
Out of Scope Changes check		The changes are limited to `MessagesTimeline.tsx` and its browser test. The stable ref callback, layout-effect recheck, and regression test directly support issue [`#26338`]. No unrelated product behavi…

Full details: Description check

Explanation

The description covers the problem, root cause, fix, linked issue, reproduction evidence, focused test results, verification limits, and blast radius. The scope section does not explicitly state maintainer approval or why the focused bug fix qualifies without it, but the linked issue and detailed scope provide sufficient context.

Full details: Out of Scope Changes check

Explanation

The changes are limited to MessagesTimeline.tsx and its browser test. The stable ref callback, layout-effect recheck, and regression test directly support issue [#26338]. No unrelated product behavior or files are changed.

Full details: Linked Issues check

Explanation

Issue [#26338] requires the “Scroll to end” pill to stay fixed while visible. The change stabilizes setTimelineList and rechecks overflow in useLayoutEffect. The regression test proves that a parent rerender no longer reports overflow as false. The PR evidence confirms a vertical jitter fix, but it does not establish that the reported horizontal movement on Linux is fixed, and no direct pill-position test covers that behavior.

Resolution

Provide reviewable evidence that the pill keeps a fixed horizontal position for the issue scenario, or add a regression test that verifies this behavior.

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Inline comments:
Review comments at
@apps/web/src/components/chat/MessagesTimeline.browser.test.tsx:
- Around line 367-368: Update the assertion after `screen.rerender(element())`
in the test to wait for the scheduled `requestAnimationFrame` measurement with
`vi.waitFor`. Require that `onContentOverflowChange` was last called with `true`
and was never called with `false`.

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Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Melv1C · 2026-10-28T09:32:13Z

Note

🤖 Responded by Claude Opus 5.5

Closing as superseded by #26375. It found the same root cause, the list ref callback re-attaching on every render, and made the same fix with useStableCallback.

This PR also re-ran the content-size check after every render, so overflow re-measures after composer inset changes and thread settling still ends after a switch. Both cases recover on their own on main: any scroll re-measures overflow, and the next content-size change ends settling. We have no concrete failure to justify the extra effect. If one turns up, it can come back as its own PR.

Copilot AI balanced review requested due to automatic review settings October 28, 2026 08:33

Copilot AI reviewed Oct 28, 2026

github-actions Bot added vouch:unvouched PR author is not yet trusted in the VOUCHED list. size:S 10-29 changed lines (additions + deletions). labels Oct 28, 2026

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Comment thread apps/web/src/components/chat/MessagesTimeline.browser.test.tsx Resolved

Melv1C and others added 2 commits October 28, 2026 09:44

test(web): overflow test waits for the re-measure after a re-render

15b075e

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Merge branch 'main' into fix/scroll-to-end-pill-jitter

d3e4f28

MarvelTiyjudy mentioned this pull request Oct 28, 2026

[Bug]: "Scroll to end" pill changes horizontal position between frames #26338

Closed

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Merge branch 'main' into fix/scroll-to-end-pill-jitter

10db3c8

Melv1C closed this Oct 28, 2026

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fix(web): scroll-to-end pill no longer jitters after scrolling up - #26365

fix(web): scroll-to-end pill no longer jitters after scrolling up#26365
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