iOS - Attachment - Chat does not scroll to bottom when sending a PDF file

[mitarachim]

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Version Number: v9.3.60-1
Reproducible in staging?: Yes
Reproducible in production?: No
If this was caught during regression testing, add the test name, ID and link from BrowserStack: https://test-management.browserstack.com/projects/2219752/test-runs/TR-19604/folder/13176710/41236942/1336303683?sort=%2Bidentifier
Email or phone of affected tester (no customers): n/a
Issue reported by: Applause Internal Team
Bug source: Exploratory - Significant User Experience Deterioration
Device used: iPhone 13 ioS 26.3.1
App Component: Chat Report View

Action Performed:

1. Open the chat
2. On a conversation, click on the + button in the compose box
3. Click on add attachment
4. Upload a PDF

Expected Result:

The chat automatically scrolls to the latest message after sending a PDF file, and the PDF preview is fully visible in the chat.

Actual Result:

Chat does not scroll to bottom when sending a PDF file

Workaround:

Unknown

Platforms:

• Android: App
• Android: mWeb Chrome
• iOS: App
• iOS: mWeb Safari
• iOS: mWeb Chrome
• Windows: Chrome
• MacOS: Chrome / Safari

Screenshots/Videos

Bug7131245_1776331850766.ScreenRecording_04-16-2026_04-28-03_1.mp4

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[melvin-bot]

Triggered auto assignment to @amyevans (DeployBlockerCash), see https://stackoverflowteams.com/c/expensify/questions/9980/ for more details.

[melvin-bot]

💬 A slack conversation has been started in #expensify-open-source

[github-actions]

👋 Friendly reminder that deploy blockers are time-sensitive ⏱ issues! Check out the open `StagingDeployCash` deploy checklist to see the list of PRs included in this release, then work quickly to do one of the following:

1. Identify the pull request that introduced this issue and revert it.
2. Find someone who can quickly fix the issue.
3. Fix the issue yourself.

[MelvinBot]

Proposal

What is the root cause of that problem?

PR #85114 (merged 2026-04-15) migrated the chat message list from InvertedFlatList to InvertedFlashList. The old iOS-specific FlatList wrapper (index.ios.tsx) had a critical guard that disabled maintainVisibleContentPosition (MVCP) while the user was scrolling to prevent it from fighting programmatic scroll calls. The new InvertedFlashList wrapper has no such guard.

When a PDF is sent, scrollToBottomForCurrentUserAction fires scrollToOffset({offset: 0}) via InteractionManager.runAfterInteractions. But FlashList's MVCP offset correction fires synchronously during layout on iOS (via native ScrollAnchor.scrollBy), overriding the async scroll-to-bottom call. The Android FlashList patch (006) has a skipNextAndroidInvertedCorrection flag for scrollToOffset, but iOS has no equivalent protection.

Additionally, the iOS-specific AttachmentModalHandler defers Navigation.goBack() via InteractionManager.runAfterInteractions, so the scroll-to-bottom may also fire while the attachment modal still covers the report screen, causing iOS to silently ignore the scroll command.

What changes do you think we should make in order to solve the problem?

Restore the iOS MVCP guard in the new InvertedFlashList component — track isScrolling state via onMomentumScrollBegin/onMomentumScrollEnd and conditionally disable MVCP during active scrolling, mirroring what the old FlatList/index.ios.tsx did.

Alternatively, add an iOS equivalent of the Android patch 006's skipNextAndroidInvertedCorrection mechanism — pause FlashList's MVCP offset correction when scrollToOffset is called programmatically.

What alternative solutions did you explore? (Optional)

Reverting PR #85114 entirely would be the quickest deploy-blocker fix but would lose the FlashList migration work. The targeted fix above preserves the migration while addressing the root cause.

Investigation details

Related issue: Expensify/App#88067 ("Blank space appears when sending a message while scrolling up") is another deploy blocker already linked to PR #85114 with the same root cause — MVCP fighting programmatic scrolling after the FlashList migration.

Data flow of the bug:

1. User sends PDF → optimistic report action added to Onyx
2. notifyNewAction → scrollToBottomForCurrentUserAction → InteractionManager.runAfterInteractions → scrollToOffset({offset: 0})
3. FlashList detects data change, MVCP fires ScrollAnchor.scrollBy(diff) synchronously on iOS
4. Native MVCP correction overrides the JS scroll-to-bottom → chat stays at old position

Key evidence:

• Old guard: src/components/FlatList/FlatList/index.ios.tsx:59 — conditionally sets maintainVisibleContentPosition to undefined during scroll
• New component: src/components/FlashList/InvertedFlashList/index.tsx — no scroll state tracking, no MVCP disabling
• Android protection: patches/@shopify/flash-list/@shopify+flash-list+2.3.0+006+fix-inverted-mvcp-android.patch — skipNextAndroidInvertedCorrection flag exists for Android only

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Next Steps for Contributor+ team: Reply with @MelvinBot implement this to create a draft PR, @MelvinBot <your feedback> to refine this analysis, or explain why you are rejecting Melvin's proposal.