fix(web): complete thread position restore only against the switched thread's rows

[saphid]

What Changed

Thread position restoration waits up to two seconds for the switched thread's saved anchor row. The deadline survives row updates, wakes the restore when it expires, and remains expired while an asynchronous fallback finishes. Manual scrolling cancels restoration.

Why

During a thread switch, the timeline can briefly show the previous thread's rows. Completing restoration against those rows can clamp the saved offset to the wrong content. The bounded wait allows the matching rows to arrive without waiting forever.

This does not fetch unloaded historical pages or guarantee exact restoration of a row that never loads. The inherited late-DOM reconciliation issue tracked in #14212 remains separate.

Verification

Integrated upstream main at 35be904. All 220 focused tests across three files passed, including delayed-anchor, deadline, and streamed-update regressions. Web typecheck, scoped lint, formatting and contribution whitespace checks passed.

Direct T3 independent review by Codex / GPT-6.1 Sol, high reasoning requested, completed with no actionable findings across the entire two-file contribution. The reviewer checked frozen identity and source; it ran no fresh tests because disk was below the reserve.

UI Changes

Recorded in the web client against an isolated dev server (vp run dev, worktree-local state, two disposable 30-turn fixture threads), headless Chromium at 1280×800. Base is upstream 35be904; candidate is this PR's head 2656db3. Both runs use the same scripted flow: open Beta, wheel up so Beta answer 24 is at the top (saved scrollTop 9025), switch to Alpha, then switch back to Beta.

To make late and missing rows reproducible, both revisions ran with the same uncommitted, dev-only harness in ChatView. For a set delay after the switch, it paints only the switched thread's newest 4 timeline entries, which don't include the saved anchor, and then the full rows. The harness is not part of this PR. GIFs are sampled at 12 fps from real-time recordings. Times come from an in-page sampler running every 100 ms (ms since the click).

Delayed anchor: the full rows arrive 1.2 s after the switch. Base restores against the 4 stale rows, marks restoration done, and lands at the end of the thread (Beta answer 30) when the real rows arrive. Candidate waits and lands on the saved Beta answer 24 at about 1.5 s.

Rows arrive at 3 s, past the 2 s wait. Candidate stops waiting at its 2 s deadline and falls back to the saved offset. When the rows arrive at 3.2 s, it does not restore late: it settles on Beta answer 30, the same position base reaches.

Gesture during the wait. A real wheel-up (−400 px) at 546 ms, with rows arriving at 1.5 s. Candidate cancels the pending restore. When the rows arrive it keeps the user's position (Beta answer 29) and does not jump to answer 24. Base completed its restore before the gesture, so it ends in the same place.

Anchor never arrives (both revisions settle identically) and source recordings

With the saved anchor never loading, both revisions fall back to the saved offset, clamped to the 4 visible entries (Beta answer 30). In the candidate, the fallback happens at the 2 s deadline instead of immediately. That timing isn't visible here because both clamp to the same spot.

Real-time MP4s: base S1 · S2 · S3; candidate S1 · S2 · S3 · S4

Not exercised in a client: citation priority, saved-end behavior, streaming during the restore, rapid multi-thread switches, desktop and mobile. The focused tests cover the deadline and streamed-update cases.

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

Original implementation: enablers/xlarge through OpenCode in T3 Code. Integration, repairs and focused checks: GPT-6 Astra in Codex/T3 Code. Independent source review: GPT-6.1 Sol in Codex/T3 Code. Client evidence: Claude Opus 5.5 in Claude Code (harness, fixtures, media) driving GPT-6 Astra in Codex CLI (browser capture).

[macroscopeapp]

Approvability

Verdict: Approved at 80babcf

Macroscope's review found this PR approvable — This is a small, self-contained web bug fix that makes thread-position restoration wait for the correct anchor row, with a bounded fallback and focused tests. It introduces no schema, deployment, security-sensitive, default-setting, or static-analysis changes.

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

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📝 Walkthrough

Walkthrough

MessagesTimeline waits up to 2,000 ms for a saved anchor row during position restoration. If the row remains absent, it restores the saved scroll offset. Cleanup clears pending timers and navigation listeners.

Changes

Thread Position Restoration

Layer / File(s)	Summary
Bounded anchor restoration apps/web/src/components/chat/MessagesTimeline.tsx, apps/web/src/components/chat/MessagesTimeline.test.tsx	Adds deadline tracking and a timer-triggered retry for missing anchor rows. After the deadline, restoration uses the saved offset without restarting the wait during a pending fallback scroll. Tests cover anchor arrival, timeout fallback, and retries after the deadline.

Priority: ⬇️ Low

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

Change: Bug fix

Suggested reviewers: maria-rcks

Merge Risk: ⚪ Minimal · up to 2656d

No concrete merge-blocking issue remains identified in the bounded restoration change. Complete the outstanding client checks before merging.

Security Architecture Review

Security architecture risk: ⚪ Minimal · up to 2656d

The change remains confined to browser-side timeline positioning. The inspected transitions do not expand thread-data access or privileges, and cancellation protects positioning state from stale asynchronous completion. This assessment does not establish overall merge readiness.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The demonstrated impact is confined to timeline positioning in the current browser component. Message-row input can affect anchor availability and scroll selection, but the changed path does not confer additional thread access, credentials, or service authority.

Trust Boundaries and Controls

• observed — Wheel, touch, pointer, and qualifying keyboard navigation cancel restoration. Cancellation marks the captured identity positioned and supersedes pending estimated scrolling with the current viewport offset. These are restoration-ownership controls, not newly introduced authorization checks.

Resilience and Maintainability Implications

• observed — Wait cleanup clears its timer and navigation listeners. Active-scroll cleanup additionally marks the effect cancelled and cancels reconciliation frames. Detachment clears the external cancellation reference only if the effect still owns it, preventing older cleanup from removing a newer owner’s callback.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly identifies the main change: thread position restoration now completes only against the switched thread's rows.
Description check	✅ Passed	The description includes the required What Changed, Why, UI Changes, and Checklist sections. It clearly explains the problem, solution, limitations, and verification results. The missing screenshots a…

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Inline comments:
In `@apps/web/src/components/chat/MessagesTimeline.tsx`:
- Around line 872-873: Update the effect containing the restoreDeadlineRef check
so the pre-deadline path schedules a timer for the remaining deadline, triggers
the restoration retry when it fires, and returns cleanup that clears the timer
and removes the listeners registered earlier in the effect. Preserve normal
restoration behavior after the deadline and ensure every invocation cleans up
its resources.

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[saphid]

Both Macroscope findings were real and are fixed in 8a5dc41:

1. Stall risk — the anchor wait returned without guaranteeing another effect run, so a thread whose rows never changed again would stay in restoring mode indefinitely (capture and end-follow suppressed). The wait now schedules a tick at the deadline that re-runs the effect, which then completes via the raw-offset fallback.
2. Unarmed cancel listeners — the wait returned before attaching the wheel/touch/pointer/keyboard listeners, so a user scroll during the wait could not cancel the eventual restore and would get yanked. The listeners now attach for the whole wait and gestures cancel through it; the deadline timer is cleared on cancel.

CodeRabbit's stability note pointed at the same two hazards; both are covered by the same commit. Verified: 62 tests passing (timelineScrollAnchoring + MessagesTimeline), pnpm tc clean, and 5/5 live scroll-switch-return cycles restoring position.

Dismissing prior approval to re-evaluate f714602

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Inline comments:
In `@apps/web/src/components/chat/MessagesTimeline.tsx`:
- Line 906: In the restore effect, keep the expired restoreDeadlineRef.current
value until fallback scrollToOffset completes; remove the expiry-path reset so
effect reruns caused by rows changes fall back immediately instead of starting
another wait. Preserve the existing deadline clears in the completion,
cancellation, and identity-reset paths.

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Dismissing prior approval to re-evaluate 80babcf

[juliusmarminge]

Note

This comment is posted by Julius' dot

The current description says the before/after images and interaction recording are still missing, and the earlier live checks predate these repairs. Closing under the verification rule. Add current-client evidence of delayed and missing anchors, the timeout, and gesture cancellation during thread switches, then request reconsideration.

[saphid]

@juliusmarminge requesting reconsideration. The description now has current-client evidence for each case you listed, recorded on 2026-10-03 against base 35be904f2f and this head 2656db3, same flow, data and viewport on both:

• Delayed anchor: base snaps to the end of the thread when the rows arrive. This head restores the saved row (S1).
• Missing anchor: both settle at the end. The head stops waiting after 2 s (S2).
• Timeout: rows arrive at 3 s, after the deadline, and the head does not jump to them late (S4, head only).
• Gesture cancellation: a wheel scroll during the wait keeps the user's position after the rows arrive (S3).

To be upfront: I couldn't trigger late anchor rows reliably on real data. Both revisions ran with the same small, uncommitted dev-only harness that withholds the older rows for a set delay after a switch. The data is two seeded 30-turn fixture threads. The description has the details, real-time MP4s, and the paths not exercised (citation priority, saved-end, streaming, rapid switches, desktop, mobile).

The branch now conflicts with main in MessagesTimeline.tsx. The restore effect on main still has the same behaviour. If you reopen, I'll rebase and recapture against the rebased head.

Client evidence: Claude Opus 5.5 in Claude Code, driving GPT-6 Astra in Codex CLI.