perf(mobile): lighter Home rows away from the viewport

[AKolenda]

What Changed

Home rows away from the viewport render a dormant frame: the same content, tap target and long-press menu inside two plain views that mirror ReanimatedSwipeable's container, with no pan gesture, Reanimated styles or hidden action buttons. The visible rows, plus two on each side, keep the full swipe row.

• swipe-row-activation.ts (new): a small store of which rows are live. HomeScreen activates the visible rows when the list loads and once any scroll settles, including status-bar, accessibility and programmatic scrolls. Changes wait until no finger is on the list, so a row never remounts under a press or long press.
• ThreadSwipeable gets a dormant prop, which thread-list-v2-items.tsx reads from that store.

Rows outside Home (the iPad sidebar) have no activation provider and stay fully live, as today. The draw distance is unchanged.

Why

On Android, a fast fling through Home shows empty space where rows should be. The list rebuilds recycled rows on the JS thread, and each row carries a pan gesture, about 15 Reanimated styles and two hidden action buttons, so the rebuild falls behind the scroll. Nobody can swipe a row while the list moves (the scroll gate already disables swipes then), so rows away from the viewport don't need that machinery.

Pixel 9, real account (57 rows on Home), release build installed in place, the same 10 fast flings per run, all runs in one session:

build	runs with blank frames	blank list time per run	longest blank	janky frames	app memory (PSS)
main (2 runs)	2 of 2	2.1 s	1.3 s	2.9–3.3%	618–629 MiB
this PR (5 runs)	1 of 5	0.31 s in that run, none in the others	0.11 s	1.9–2.4%	525–571 MiB

Memory goes down (mean 624 → 550 MiB) because only the rows on screen plus two on each side carry the full swipe tree, instead of every mounted row.

Checked on the device: swipe to reveal actions after the list has settled and on rows revealed by a fling, tap to open right after a fling, and the long-press menu right after returning from a thread. iOS uses the same components but was not tested; I only had an Android device.

UI Changes

No visual change to a row. Same flings, main on the left, this PR on the right, using this PR's worst run of five (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

Summary by CodeRabbit

• Improvements
  • Thread list scrolling and swipe interactions are more responsive. Swipe actions become available for nearby rows as you scroll, while active touches are preserved without interruption.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This production mobile change introduces viewport-aware row activation with new shared state, timers, touch deferral, and an alternate swipe-rendering path across the Home list. Its interaction-sensitive runtime impact is broader than a small self-contained optimization and should receive human review.

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[coderabbitai]

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📝 Walkthrough

Walkthrough

The home thread list now controls which rows have active swipe behavior. It updates activation state from list visibility, scroll, swipe, and touch events. Thread rows use that state to render swipeable or dormant content.

Changes

Swipe-row activation

Layer / File(s)	Summary
Activation store and hook apps/mobile/src/features/home/swipe-row-activation.ts, apps/mobile/src/features/home/swipe-row-activation.test.ts	The store tracks active row keys, avoids notifications when the active set is unchanged, and defers activation updates during touch. The hook reports whether a keyed row is dormant. Tests cover set equality, deferred updates, and unsubscription.
Home list activation apps/mobile/src/features/home/HomeScreen.tsx	The list activates rows on load, after scrolling settles, and when swipe is enabled. It tracks touch state and continues forwarding scroll events to the material FAB handler.
Dormant row rendering apps/mobile/src/features/home/thread-swipe-actions.tsx, apps/mobile/src/features/threads/thread-list-v2-items.tsx	The provider supplies activation state to descendants. Thread rows use their activation key to select dormant rendering, which displays children without swipe machinery and supplies a no-op close callback.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Refactor

Sequence Diagram(s)

sequenceDiagram
  participant LegendList
  participant SwipeRowActivation
  participant ThreadListV2Row
  participant ThreadSwipeable
  LegendList->>SwipeRowActivation: Update active keys and touch state
  ThreadListV2Row->>SwipeRowActivation: Read dormancy for activationKey
  ThreadListV2Row->>ThreadSwipeable: Pass dormant state

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Suggested reviewers: juliusmarminge

Merge Risk: 🔵 Low · up to b2ac5

In a multi-touch edge case, newly visible Home rows can remain without swipe actions until another list touch ends. This is a bounded issue to fix or explicitly accept before merging.

Security Architecture Review

Security architecture risk: 🔵 Low · up to b2ac5

The change appears limited to swipe-row presentation and interaction timing. Thread actions remain connected to their existing handlers, but an unusual scroll during a held long press leaves some uncertainty about whether an open menu remains available after release.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The identified change is confined to Home row gesture availability and rendering; the supplied impact map identifies one changed mobile system and no changed dependency edges.

Trust Boundaries and Controls

• observed — The scroll gate controls swipe gesture availability, while row selection and long-press menu actions retain their separate row callbacks. No authorization decision is added to the activation store.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description includes all required sections: What Changed, Why, UI Changes, and Checklist. It explains the implementation, motivation, test results, platform coverage, and interaction checks. The U…
Title check	✅ Passed	The title clearly identifies the mobile performance change: Home rows outside the viewport use a lighter rendering path. It is concise and directly related to the main changeset.
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.

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[coderabbitai]

Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
In `@apps/mobile/src/features/home/HomeScreen.tsx`:
- Line 999: Update the touch handlers using swipeRowActivation to track
identifiers for touches that start on the list, and set touching from whether
any tracked list touch remains active. Do not use the screen-wide
nativeEvent.touches count, so touches on outside controls cannot keep list
activation updates deferred.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository: pingdotgg/t3code/.coderabbit.yaml

Review profile: CHILL

Plan: Advanced

Run ID: 0c437d3a-353f-444f-9c39-fb63a4d83d33

📥 Commits

Reviewing files that changed from the base of the PR and between 4293433 and b2ac57b.

📒 Files selected for processing (5)

• apps/mobile/src/features/home/HomeScreen.tsx
• apps/mobile/src/features/home/swipe-row-activation.test.ts
• apps/mobile/src/features/home/swipe-row-activation.ts
• apps/mobile/src/features/home/thread-swipe-actions.tsx
• apps/mobile/src/features/threads/thread-list-v2-items.tsx

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[AKolenda]

Superseded by #13702 (same change plus the fix for CodeRabbit's touch finding).