perf: prevent iOS OOM kill with download concurrency limiter and reduced rebuilds

[9clg6]

Summary

Prevent iOS out-of-memory kills caused by unbounded concurrent image downloads when scrolling fast through rooms with many images.

Changes

• Add ImageDownloadQueue (semaphore pattern, max 4 concurrent): Throttles simultaneous image download/decode operations. Each MxcImage acquires a slot before loading and releases it when done or disposed.
• Cancel pending downloads on dispose: When MxcImage is scrolled off-screen, its download ticket is cancelled, preventing wasted work and memory allocation for images the user will never see.
• Integrate queue into GetPreviewUrlMixin: Throttles preview_url HTTP requests through the same shared queue, preventing dozens of URL preview fetches from firing simultaneously.
• Add didUpdateWidget early-return in ChatList and AppAdaptiveScaffoldBody: Skips redundant processing when args haven't changed, avoiding unnecessary rebuilds on every parent rebuild.
• Bump file_picker 10.1.7 → 10.3.10: Fixes ObjC FileUtils class name collision with OSAnalytics that caused random native crashes.
• 6 unit tests for ImageDownloadQueue (FIFO ordering, cancel, release).

Performance measurements

Measured with Flutter Web --profile, CPU throttling 4x, Chrome DevTools trace, ~75s navigation (DMs, groups 51-85 members, contacts, settings, scrolling, member lists). Fresh Chrome instance.

Metric	main	image-throttle	Delta
JS Heap delta	+73.39 MB	+27.19 MB	-62.9%
JS Heap end	170.94 MB	126.59 MB	-26.0%
Long tasks (>50ms)	51	38	-25.5%
GC events	5,538	1,953	-64.7%
GC total time	1,702 ms	1,556 ms	-8.6%

The primary benefit is on iOS where the OOM killer is aggressive. The web measurements confirm reduced memory pressure. On iOS, this prevents the app from being killed during fast scrolling through image-heavy rooms.

Raw trace file attached: perf-trace-image-throttle.json.gz (load in Chrome DevTools > Performance > Load profile to verify).

Test recommendation

1. Open a room with many images (e.g., Bug Me Harder)
2. Scroll quickly up and down
3. On iOS: the app should NOT be killed by the OS
4. On web: monitor DevTools Memory — concurrent downloads should stay at ≤4
5. Run flutter test test/utils/image_download_queue_test.dart

Ticket

No ticket — discovered during performance profiling.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 072fa0f4-6315-4b2c-93dd-3193fc674207

📥 Commits

Reviewing files that changed from the base of the PR and between 844b35b and ec6c684.

⛔ Files ignored due to path filters (1)

• pubspec.lock is excluded by !**/*.lock

📒 Files selected for processing (6)

• lib/pages/chat_list/chat_list.dart
• lib/utils/image_download_queue.dart
• lib/widgets/layouts/adaptive_layout/app_adaptive_scaffold_body.dart
• lib/widgets/mixins/get_preview_url_mixin.dart
• pubspec.yaml
• test/utils/image_download_queue_test.dart

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch perf/image-download-throttle

📝 Coding Plan

• Generate coding plan for human review comments

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[github-actions]

This PR has been deployed to https://linagora.github.io/twake-on-matrix/2950