fix(mobile): prevent Android compose FAB animation jitter

[PixPMusic]

The Android New thread FAB jumps sideways while shrinking and expanding on scroll. Its native width animation was repeatedly resizing the right-aligned React Native host.

Keep the host width stable and animate the button, label, and native vector icon together inside Compose. A separate press target leaves the list beside the button tappable. Only the scrolling Android compose FAB changes; other FABs and iOS are unchanged.

Small follow-up to #11841, which merged before this fix was ready.

Before / after

Actual Android 17 cover display, same Material You dark theme, 100% system text and disposable showcase data. Normal-speed recordings: scroll down to shrink, then up to expand.

Before — merged main (9686cd9afe):

fab-main-before.mp4

After — this PR:

fab-followup-after.mp4

Verification

• Four existing scroll-direction tests, mobile typecheck, scoped lint and formatting.
• Android debug build/install; both scroll directions, compose activation in both sizes, and list taps beside the FAB.
• Earlier verification of this same fix covered 150% system text. No iOS changes or new iOS test pass.

Model: GPT-5 (Codex). Harness: Codex in T3 Code.

Summary by CodeRabbit

• New Features
  • Added an Android “New thread” compose button with a pencil icon and expandable layout.
  • The button adapts its appearance to the current theme, supports accessibility-friendly touch targets, and provides Android ripple feedback.
  • Updated the extended new-thread action to use the new compose button when expansion is available.

[macroscopeapp]

Approvability

Verdict: Approved at ad654e2

Macroscope's review found this PR approvable — This is an isolated Android FAB animation fix that keeps the existing host stable and moves expansion rendering into Compose, while preserving other platforms and button paths. The added native UI and touch-target logic are confined to the affected component and do not introduce broader product or infrastructure changes.

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[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

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Review profile: CHILL

Plan: Advanced

Run ID: 3f0a2543-6d22-4685-a8b7-048b02e033fe

📥 Commits

Reviewing files that changed from the base of the PR and between e47a11e and ad654e2.

📒 Files selected for processing (3)

• apps/mobile/assets/icons/compose.xml
• apps/mobile/src/components/MaterialNewThreadButton.android.tsx
• apps/mobile/src/components/MaterialScrollComposeButton.android.tsx

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📝 Walkthrough

Walkthrough

Android adds a compose icon and an expandable compose button. MaterialNewThreadButton selects it for extended buttons with defined expansion state. Other cases keep the floating action button. The button tracks width and forwards presses through an accessible target.

Changes

Android compose button

Layer / File(s)	Summary
Compose button implementation apps/mobile/assets/icons/compose.xml, apps/mobile/src/components/MaterialScrollComposeButton.android.tsx	The Android compose button uses a new vector icon, renders the “New thread” label, tracks expanded width, and forwards presses through an accessible target.
New-thread button wiring apps/mobile/src/components/MaterialNewThreadButton.android.tsx	MaterialNewThreadButton renders MaterialScrollComposeButton when extended is true and expanded is defined. Other cases retain MaterialFloatingActionButton.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant MaterialNewThreadButton
  participant MaterialScrollComposeButton
  participant ExtendedFloatingActionButton
  participant PressTarget
  MaterialNewThreadButton->>MaterialScrollComposeButton: render eligible extended state
  MaterialScrollComposeButton->>ExtendedFloatingActionButton: render compose icon and New thread label
  MaterialScrollComposeButton->>PressTarget: apply measured or collapsed width
  PressTarget->>MaterialScrollComposeButton: invoke onPress

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Merge Risk: ⚪ Minimal · up to ad654

The Android FAB change has no identified current-head issue and is ready to merge.

🚥 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 2 functions across 2 files. (1 skipped: 1…	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 identifies the Android Compose FAB animation jitter fix and matches the main change.
Description check	✅ Passed	The description explains what changed, why it changed, the UI behavior before and after, and the verification performed. It includes recordings for the interaction change, although it does not reprodu…
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.

Full details: Docstring Coverage

Explanation

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 2 functions across 2 files. (1 skipped: 1 unsupported.)

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