perf(web): diff panel no longer re-renders every file header each time a patch arrives

[stevan-borus]

Summary

Scrolling the Diff panel on a large change set stutters while files are still loading, and collapsing files while scrolling makes it worse. The progressive loader delivers patches four at a time as the reader scrolls. Each arrival changes one viewer item, and the viewer's portal slots then re-render the app-owned header content of every visible file: a Base UI tooltip, a button, a copy control and a status per header, 30 to 40 headers at a time. On a real 139-file diff a CPU profile puts about 40 ms of main-thread work per arrival in those headers; patch parsing for the whole scroll is 11 ms.

Fix: the three header render callbacks now return small components (DiffFileCollapseToggle, DiffFileHeaderSuffix, DiffFileHeaderStat) fed only primitives and stable callbacks. With the React Compiler on in apps/web, a component whose props did not change returns its cached elements, so React skips the Base UI subtree for every header except the one whose file changed. JSX written inline in a callback that the viewer invokes per item has no such scope, which is why the inline version re-rendered everything. No React.memo: measured with and without it, it adds nothing here. To make the props stable, toggleDiffFileCollapsed and retry read their changing inputs through refs published in layout effects, fileStates is memoized on its inputs, and beginComment in AnnotatableCodeView no longer changes identity with the file list, which also stops the viewer from treating its options as changed on every arrival. No behavior change.

Submitted under the small-focused-fix exception: no new behavior or settings, three files, and the defect is a measurable stall on the default path.

Test plan

Measured on a real 139-file working-tree diff from a TypeScript repo (417 kB), dev build, same main commit, driving the Diff panel's scroll container in rAF steps while patches arrive and recording long-animation-frame entries. Two runs each in headless and in headed Chromium, then one pass inside T3's own Browser panel.

default collapsed list, scrolling while loading	before	after
websocket handler per arrival (longest)	43 ms	20 to 27 ms
p90 frame	29 to 33 ms	19 to 21 ms
worst frame	98 to 116 ms	63 to 75 ms
long-animation-frame total	333 to 392 ms	202 to 310 ms
time to complete the pass	2.7 s	2.3 s

Inside the T3 Browser panel on the same thread (narrower panel, 24 headers in view): before, worst frame 81 ms with four frames over 50 ms and 250 ms of long animation frames; after, worst frame 43 ms, no frame over 50 ms, no long animation frames.

files open, collapsing one while scrolling	before	after
typical collapse frame	21 ms	17 ms
worst frame	51 to 66 ms	31 to 40 ms

Profile frames before the change, inclusive over one pass: SlotPortals 262 ms, FastComponent (Base UI) 294 ms, TooltipTrigger 154 ms, TooltipRoot 102 ms, renderHeaderPrefix 92 ms, Button 92 ms, DiffPanel 135 ms.

• tsc --noEmit in apps/web: clean.
• vp test run on StyledDiffCodeView.test.tsx, diffRendering.test.ts, diffCollapse.test.ts: 32 passed.
• Scoped lint: only the two pre-existing warnings in DiffPanel.tsx. The refs behind the stable callbacks are assigned in layout effects, so handlers only see committed inputs.
• No new tests: the change is a render-count property with no seam in this tree that would not simply mirror the implementation, so the evidence is the measurement above.
• No visual change, so no screenshots. Mobile uses its own native review adapter and is not affected.

Implementation and verification: Claude Fable 5.1 via Claude Code.

[macroscopeapp]

Approvability

Verdict: Approved at 2ed4503

Macroscope's review found this PR approvable — This is a focused web performance fix that memoizes existing diff-header controls and stabilizes callbacks without adding capabilities, changing defaults, or altering APIs. Its runtime effect is limited to reducing unnecessary renders during progressive patch loading.

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📥 Commits

Reviewing files that changed from the base of the PR and between 6365fe7 and a65ece4.

📒 Files selected for processing (3)

• apps/web/src/components/DiffPanel.tsx
• apps/web/src/components/diffs/AnnotatableCodeView.tsx
• apps/web/src/components/diffs/useReviewFilePatches.ts

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📝 Walkthrough

Walkthrough

The changes update callback data access in diff review components, memoize per-file state, and extract components for diff headers. Callbacks read current values from refs.

Changes

Diff review rendering

Layer / File(s)	Summary
Stabilize review-hook data access apps/web/src/components/diffs/AnnotatableCodeView.tsx, apps/web/src/components/diffs/useReviewFilePatches.ts	beginComment and retry read current data from refs instead of captured callback dependencies. fileStates is memoized from files and patches; its error and truncation checks remain unchanged.
Extract diff-header controls and rendering apps/web/src/components/DiffPanel.tsx	Extracted components render header suffixes, statistics, and collapse controls. The collapse callback reads the current scope and default collapsed keys from a ref.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Refactor

Suggested reviewers: maria-rcks

Merge Risk: ⚪ Minimal · up to a65ec

No actionable regression is established in the reviewed changes; the PR appears ready to merge.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 6365f

The changes remain concentrated in the diff-review UI. A rendering interruption could make a retry act on a comparison different from the one displayed. Existing environment-specific query handling limits the consequences; no access-control bypass or data disclosure was demonstrated.

Retained concerns

• Low · architecture · inferred: Stable retry and collapse handlers consume scope inputs assigned during render without a commit/version fence. If a render is interrupted while the panel instance remains mounted, an action from the displayed comparison can refresh a query or update collapse ownership for an uncommitted comparison. Query identity and environment-session controls remain intact, so the supported concern is scope consistency and failure containment, not an authorization bypass. Runtime occurrence has not been reproduced.

Security review details

Security Blast Radius

• inferred — The supported exposure is a user-triggered refresh of an existing environment-scoped diff query, potentially for a comparison other than the displayed one under interrupted rendering. The inspected changes do not establish attacker-selected credentials, additional environment access, or a cross-tenant disclosure path.

Trust Boundaries and Controls

• observed — The existing query factory preserves the selected query’s environment identity. Execution requires a connected environment with a session; connecting or backoff states wait, and offline or blocked states fail. Diff-patch reads also retain a four-permit concurrency bound. These are counterevidence to privilege expansion, not proof of end-to-end authorization coverage.

Hardening Proposals

• proposed — Preserve stable callback identity with commit-synchronized inputs or explicit scope/version tokens, so displayed UI events cannot consume an abandoned render’s scope. This would strengthen transition ownership without changing the existing environment-query boundary.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 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	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 describes the main change: reducing Diff panel header re-renders when patches arrive. It is specific and uses a conventional commit-style prefix.
Description check	✅ Passed	The description covers the problem, the change, the small-focused-fix scope rationale, and detailed verification results. It uses “Summary” and “Test plan” instead of the template’s exact headings, bu…

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Inline comments:
Review comments at @apps/web/src/components/diffs/AnnotatableCodeView.tsx:
- Line 136: Move render-time ref updates to layout effects so handlers only read
values from committed renders: in AnnotatableCodeView, publish the committed
file map; in useReviewFilePatches, publish committed queries and files; and in
DiffPanel, publish committed scope defaults. Update each affected location:
apps/web/src/components/diffs/AnnotatableCodeView.tsx lines 136-136,
apps/web/src/components/diffs/useReviewFilePatches.ts lines 150-150, and
apps/web/src/components/DiffPanel.tsx lines 681-681.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7812230 and 2ed4503.

📒 Files selected for processing (3)

• apps/web/src/components/DiffPanel.tsx
• apps/web/src/components/diffs/AnnotatableCodeView.tsx
• apps/web/src/components/diffs/useReviewFilePatches.ts

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 8 remain after this review.

[juliusmarminge]

Thanks for this. I rebased it onto main and pushed two changes to your branch:

• Rebase: the only conflict was the effect/unstable/reactivity → effect/reactivity import path after the Effect 4.0.1 upgrade. All three of your commits are kept.
• f109eed, two simplifications from review:
  • DiffFileHeaderStat is gone. DiffStatLabel is already memo on its two numbers, so the wrapper added nothing.
  • beginComment reads the file diff and path from the context.item it is handed instead of filesByKey. That keeps the callback stable without the ref and its layout effect.

GPT 6.1 Sol and Claude Fable 5.1 reviewed it. Both confirmed the header components compile and cache under the React Compiler, the props are stable on each arrival, and there's no behaviour change.

The scope-reset bug from the original report is a separate cause, fixed in #16571.