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 every finger that started on the list has lifted, so a row never remounts under a press or long press; a finger resting on another control does not hold them.
• 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 swipe actions are now active for rows near the visible area of the list, rather than throughout the list.
  • Swipe-action availability updates as you scroll, while respecting touches already in progress to avoid disrupting gestures.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR introduces asynchronous viewport activation, touch tracking, and an alternate dormant rendering tree for existing Home rows, changing their gesture and remount lifecycle across several production components. The interaction-sensitive behavior spans list recycling and platform touch/scroll handling, so it warrants human review.

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📝 Walkthrough

Walkthrough

The Home screen now tracks list touches and activates rows near the visible range. Thread rows use activation state to render either the existing swipeable path or a dormant path that skips swipeable machinery.

Changes

Swipe row activation

Layer / File(s)	Summary
Activation state and touch tracking apps/mobile/src/features/home/swipe-row-activation.ts, apps/mobile/src/features/home/swipe-row-activation.test.ts	The activation store tracks active row keys and list touches. It defers updates while tracked touches remain and notifies subscribers when the active set changes. Tests cover activation changes, touch tracking, and unsubscribe behavior.
Dormant row integration apps/mobile/src/features/home/thread-swipe-actions.tsx, apps/mobile/src/features/threads/thread-list-v2-items.tsx	The provider supplies the activation store. Thread rows use their activation keys to determine whether ThreadSwipeable renders its dormant path.
Home list activation and touch tracking apps/mobile/src/features/home/HomeScreen.tsx	The Home screen activates rows around the visible range, tracks list touches, passes row keys to thread rows, and continues forwarding scroll events to the FAB handler.

Priority: ➖ Normal

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

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant HomeScreen
  participant SwipeRowActivation
  participant ThreadListV2Row
  participant ThreadSwipeable
  HomeScreen->>SwipeRowActivation: activate row keys and track list touches
  SwipeRowActivation-->>ThreadListV2Row: notify activation changes
  ThreadListV2Row->>SwipeRowActivation: read dormant state for activation key
  ThreadListV2Row->>ThreadSwipeable: pass dormant state

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Suggested reviewers: juliusmarminge

Merge Risk: 🟡 Moderate · up to 30aaa

The mobile typecheck is blocked by the new touch-tracking code. Correct the identifier types before merging.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 30aaa

Home rows now switch between lightweight and swipe-enabled rendering. Thread actions appear to retain their existing identity and action paths, but touch-transition behavior has not been verified across devices.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The changed activation decision affects Home thread-row interactions; the optional provider leaves consumers without Home activation on the live path. The inspected change does not establish a new service or credential boundary.

Trust Boundaries and Controls

• inferred — Activation selects a rendering mode rather than a thread action or its authority. The inspected dormant branch supplies the same row children, while thread-bound callbacks remain in the row.

Resilience and Maintainability Implications

• inferred — The no-remount-during-press guarantee depends on list touch-start and cancellation delivery matching the store’s tracked-touch model. The source and synthetic tests do not resolve platform event ordering.

Hardening Proposals

• proposed — Validate concurrent touches, cancellation, and responder transfers on Android and iOS devices before relying on the no-remount-during-press behavior.

🚥 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
Title check	✅ Passed	The title clearly and concisely describes the main change: reducing the work performed by Home rows outside the viewport.
Description check	✅ Passed	The description includes the required What Changed, Why, UI Changes, and Checklist sections. It explains the implementation, motivation, test results, interaction checks, and includes visual evidence …
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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Actionable comments posted: 1

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Inline comments:
In `@apps/mobile/src/features/home/swipe-row-activation.ts`:
- Line 38: Update the touch identifier types used by `trackTouches` and its
associated store state to `number` so they accept identifiers passed directly
from `HomeScreen.trackListTouches`; update the related test fixtures to use
numeric identifiers.

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ℹ️ Review info

⚙️ Run configuration

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Run ID: dd12d332-8932-48a3-bca4-f62ac60523ad

📥 Commits

Reviewing files that changed from the base of the PR and between 2598ce7 and 30aaa9d.

📒 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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