perf(web): returning to a thread reuses its measured row sizes

[flamboh]

Note

🤖 Claude Opus 5.5 on behalf of Oliver

Problem

Returning to a thread (for example, closing Settings) rebuilds its timeline from a fixed 90 px row estimate, while real rows are often 90 to 400 px. LegendList mounts a much larger row pool than needed, and the list mounts before the chat canvas has reported its width, causing extra synchronous layouts.

Fix

• When leaving a thread, store the median measured size of its rows, plus the chat canvas width, in the existing per-thread scroll position cache. The median counts only that thread's rows, because the retained list keeps measurements from threads visited earlier. It is computed once on departure, not on every scroll. A median keeps a few tall code blocks from inflating the hint. An inflated hint would mount too few rows.
• When returning to a thread that was measured before, wait for the canvas width, then use that median as the size hint, but only at the same canvas width. Saving and comparing the same clientWidth keeps fractional zoom from silently disabling reuse. Unmeasured threads mount immediately as before.
• Keep mounted timelines alive during panel maximization.

Return to thread from Settings	Before → after (CPU task time)
Production, repeat	1140 → 906 ms
Production, first	1251 → 937 ms
Dev (unthrottled), repeat	859 → 711 ms

Production is 4× throttled; dev is unthrottled. Fixture: 24 idle threads, 160 messages in the long thread. Production cold-return React commits rose from 19 to 20 even as task time and layouts fell. These numbers were measured with the earlier mean-based hint, before the review changes above.

Scope and approval

There is no prior issue or discussion for this. It is a focused performance improvement with no intended behavior change: sizes are only a startup hint, real measurement still wins, and a width change falls back to the original estimate. CONTRIBUTING lists performance improvements among the changes most likely to be accepted. No maintainer has approved it.

Merge note

Lands after #17348. Whoever merges second resolves two adjacent-insertion conflicts. In MessagesTimeline.tsx, keep both new layout effects after the handleScroll frame effect. In MessagesTimeline.test.tsx, keep both new test blocks whole. Git interleaves those hunks, so take one side and paste the other test in. #17348 also adds a liveFollowEnabled argument to rememberTimelinePosition, so this PR's median test needs false as that third argument to typecheck. With that, the combined result passes both test files and tsc.

Before and after

In a 160-message thread scrolled to the middle, opening Settings and going Back. Both sides return to the same message. In this take, the return painted in 1,377 → 974 ms and settled in 1,770 → 1,142 ms.

https://gh-file-drop-api-prod-galwoqjslzlnws6s.oliver-boorstein.workers.dev/f/44b63c3557a0af7f/pr17123-before-after.mp4

Production build, CPU throttled 4× during the interaction, headless Chromium. Before is main, after is this branch at the time of recording. These are single takes, so the timings are illustrative; the table above uses medians.

Validation

• vp test run on MessagesTimeline.test.tsx and timelineScrollAnchoring.test.tsx: 105 passed. This includes a test that leaving a thread saves the median of its rows at the canvas width, and that fails without the departure save.
• Web typecheck and scoped lint (no errors) pass.
• Earlier headless dev and production checks: scroll position restored at the bottom and mid-thread, after Escape and Back. Not exercised: active provider turns, native Electron, and re-measuring after the review changes. Mobile does not use this timeline.

Implemented by GPT-6.1 Sol (Codex harness, delegated through T3 Code); split, verified, opened, and revised after review by Claude Opus 5.5 (Claude Code in T3 Code).

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — The change alters the production timeline’s return path by delaying list mounting until canvas sizing is ready and reusing cached row estimates to affect virtualization and scroll restoration. It is localized and tested, but the automatic gate controls substantial rendering and layout work, warranting human review.

^{You can add or adjust custom eligibility rules. Learn more.}

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📥 Commits

Reviewing files that changed from the base of the PR and between 32c4342 and d9b7356.

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📒 Files selected for processing (4)

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

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📝 Walkthrough

Walkthrough

The timeline stores a median measured row size with the viewport width. It uses canvas width and viewport readiness to select an estimated row size and control restoration, overflow measurement, and rendering.

Changes

Timeline sizing and restoration

Layer / File(s)	Summary
Store viewport-specific row sizing apps/web/src/components/chat/timelineScrollAnchoring.ts, apps/web/src/components/chat/timelineScrollAnchoring.test.tsx	Remembered positions can store the viewport width and median measured row size. The median uses measurements for current rows. Tests cover unrelated rows, tall rows, and rows without measurements.
Use readiness for sizing and rendering apps/web/src/components/chat/MessagesTimeline.tsx, apps/web/src/components/chat/MessagesTimeline.test.tsx	The timeline uses the saved median when its viewport width matches the canvas width; otherwise, it uses an estimate of 90. Readiness gates restoration, overflow measurement, and rendering. On cleanup, the timeline saves the departing thread’s median when the viewport width and measurements are available. A test verifies the saved width and median.

Priority: ⬇️ Low

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

Change: Other

Suggested reviewers: t3dotgg

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Merge Risk: 🔵 Low · up to d9b73

Returning to a thread can briefly use the old row-size estimate, reducing the intended speed improvement. The change is mergeable with this bounded performance risk understood.

Security Architecture Review

Security architecture risk: ⚪ Minimal · up to d9b73

The change remains within local chat rendering and session-only layout state. The reviewed changes introduce no material security risk or expansion of permissions or data exposure.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The incremental exposure is browser-session timeline layout and restoration state. Scoped thread keys and per-row measurement selection bound ownership; the new field carries geometry rather than message contents, credentials, or authority. The inspected production caller does not turn the flagged UI entrypoints into remotely callable operations.

Trust Boundaries and Controls

• observed — ChatCanvasContext's relevant callbacks mutate local layout and DOM-registration state. The new sizing path neither invokes nor widens the component's pre-existing shell-command or message-sending callbacks. The full comparison adds geometry handling, not a new privileged dispatch path.

Resilience and Maintainability Implications

• observed — The cache extension preserves existing scroll and disclosure fields, does nothing for an absent position, and skips saving when measurements or width are unavailable. Width mismatch falls back to the prior estimate. These behaviors contain incomplete sizing information within local rendering rather than making it an authority-bearing state transition.

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Pre-merge checks | 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely describes the main change: reusing measured row sizes when returning to a thread.
Description check		The description is mostly complete. It explains the problem, implementation, scope rationale, performance results, validation, limitations, and UI recording. It uses “Fix” and “Validation” instead of …

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @apps/web/src/components/chat/MessagesTimeline.tsx:
- Line 608: Update the timelineReady condition in MessagesTimeline so a retained
timelineViewportElement does not bypass the canvas.container.width check when
switching threads. Wait for the returning thread’s canvas width to be available
before mounting the list with saved sizes.

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[flamboh]

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