fix(mobile): scroll to end jumps instead of glides when Reduce Motion is on

[saphid]

Problem

With iOS Reduce Motion on, a thread's Scroll to end button still glides through the whole conversation. After you send a message, the new message still glides into place. Part of #15953, which has repro steps on main a1d9d72aef.

Change

ThreadDetailScreen keeps the system Reduce Motion value in a ref. It queries AccessibilityInfo.isReduceMotionEnabled() on mount, and a reduceMotionChanged listener keeps it current. Both calls to scrollMessageToEnd pass animated: !reduceMotionRef.current. The ref starts as true until the first query resolves, so an early scroll never animates against the user's setting. A change event, or the effect's cleanup, marks a still-pending first query as stale, so it cannot overwrite a newer value. The keyboard-dismissal ordering before the post-send scroll is unchanged.

Scope and approval

A small, focused fix for an accessibility bug reported in #15953. It replaces the mobile half of #12871, which was closed for missing live motion verification. The web half is a separate PR, #16014.

Verification

• vp run typecheck in apps/mobile, plus scoped vp lint and vp fmt --check: pass on the head commit. The simulator recordings below were made on 941f22f. The follow-up commit 0a76661 only stops a stale first query from overwriting a newer value, so the recorded paths are unchanged. No unit test: the change only sets a flag inside a large screen component, so the proof is the recordings below.
• iOS 26.5 simulator (iPhone 17 Pro), T3 Code Dev built from this checkout, isolated dev backend. The real Settings → Accessibility → Motion → Reduce Motion switch was used, toggled live while the thread stayed open. Screen recorded with simctl io recordVideo; significant frame changes counted with ffmpeg mpdecimate.

Action	main, Reduce Motion on	this PR, on	this PR, switched off live	this PR, switched back on live
Scroll to end	19 (glide)	3 (jump)	21 (glide)	4 (jump)
Send a message	jumps toward the end, then glides the new message up	waits for the keyboard to close, then one jump to the same anchor	glide	—

Scroll to end, Reduce Motion on, changed frames:

main	this PR

Recordings:

• main, on: Scroll to end, send
• this PR, on: Scroll to end, send
• this PR, switched off live: Scroll to end, send
• this PR, switched back on live: Scroll to end

Not verified:

• Android. No emulator was available. isReduceMotionEnabled follows Android's "Remove animations" setting there.
• A physical iPhone.
• Keyboard animation itself is system-owned and unchanged.

Reproduction, implementation and evidence: Claude Opus 5.5 in Claude Code, inside T3 Code. Independent review: GPT-6.1 Sol through Codex in T3 Code.

🤖 Generated with Claude Code

[macroscopeapp]

Approvability

Verdict: Approved at 0a76661

Macroscope's review found this PR approvable — This is a narrowly scoped accessibility bug fix that conditionally disables animation for existing scroll actions when the system requests reduced motion, while preserving normal behavior and cleanup semantics. It introduces no schema, infrastructure, security-sensitive, or static-analysis changes.

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

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⚙️ Run configuration

• Configuration used: Repository: pingdotgg/t3code/.coderabbit.yaml
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📥 Commits

Reviewing files that changed from the base of the PR and between 941f22f and 0a76661.

📒 Files selected for processing (1)

• apps/mobile/src/features/threads/ThreadDetailScreen.tsx

📝 Walkthrough

Walkthrough

The thread detail screen now reads and observes the system Reduce Motion setting. Submission-anchor and manual scroll-to-end actions animate only when Reduce Motion is disabled.

Changes

Thread scrolling accessibility

Layer / File(s)	Summary
Reduce Motion-aware thread scrolling apps/mobile/src/features/threads/ThreadDetailScreen.tsx	The screen reads the Reduce Motion setting on mount and updates it when the setting changes. Submission-anchor and manual scroll-to-end scrolling use animation only when Reduce Motion is disabled. Both actions continue to keep the keyboard open. The manual scroll rejection handler still unfreezes scrolling.

Priority: ⬇️ Low

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

Change: Bug fix

Suggested reviewers: juliusmarminge

Merge Risk: 🔵 Low · up to 941f2

On Android, a Reduce Motion change during screen initialization can leave thread scrolling using the old preference for the rest of that screen session. The timing is narrow, so the PR is mergeable with this bounded accessibility issue noted.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1…
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 summarizes the main change: scrolling jumps instead of animating when Reduce Motion is enabled.
Description check	✅ Passed	The description covers the problem, change, scope, and verification. It includes focused test results, detailed iOS simulator evidence, and clear notes about unverified platforms and behaviors.

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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:
Review comments at @apps/mobile/src/features/threads/ThreadDetailScreen.tsx:
- Line 688: Guard the initial AccessibilityInfo query in the useEffect that
updates reduceMotionRef so its result is ignored after the reduceMotionChanged
listener has received an event. Keep listener events authoritative by marking
that an event arrived and continuing to update reduceMotionRef from the
listener.

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Reviewing files that changed from the base of the PR and between 7812230 and 941f22f.

📒 Files selected for processing (1)

• apps/mobile/src/features/threads/ThreadDetailScreen.tsx

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[saphid]

Review requested

Date (UTC)	Reviewer	Where
2026-10-06	Julius	Discord DM

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