[0.88 regression] iOS: maintainVisibleContentPosition skips the correction when removeClippedSubviews has detached the anchor #58910

Description

Since #57294 (0.88.0-rc.0), a ScrollView or FlatList with both maintainVisibleContentPosition and removeClippedSubviews no longer corrects for a prepend taller than about a screen. The visible content is pushed down by the prepend's full height. 0.87.1 holds it.

In the repro, a ScrollView at y=2000 gets 12 rows of 200pt prepended:

Cause

#57294 added this check to _adjustForMaintainVisibleContentPosition (L1114-L1118):

if (_firstVisibleView.superview != _contentView) {
  return;
}

-mountingTransactionDidMount runs -_remountChildren before the adjustment. With removeClippedSubviews, a prepend moves the anchor out of the clip rect, so -_remountChildren detaches it, and the check returns. But the detached anchor is still a valid anchor: the content view still holds it in _reactSubviews, and it already has the new frame. That frame is what 0.87.1 corrects with.

The check isn't needed to catch deleted views. The registry resets a view's tag to 0 when it's enqueued (RCTComponentViewRegistry.mm#L67-L68), so the tag check just above already returns for those.

Fix

Remove the check. PR to follow.

Steps to reproduce

1. git clone https://github.com/mozzius/scrollview-mvcp-anchor-repro && cd scrollview-mvcp-anchor-repro/ReproducerApp && yarn install
2. cd ios && bundle install && RCT_USE_PREBUILT_RNCORE=0 bundle exec pod install && cd .. (core must be built from source for the repro's patch to compile in)
3. yarn start, then yarn ios
4. Open B: unmounted, turn Clip on, set Guard to main, and tap Prepend. It reports +2400.0pt: JUMPED.
5. Set Guard to 0.87.1 or fix and tap Prepend. It reports +0.0pt: held.

The repro's App.tsx in an unmodified 0.88.0-rc.4 app shows the same jump at step 4.

React Native Version

0.88.0-rc.4 and main. Not 0.87.1.

Affected Platforms

Runtime - iOS

Output of npx @react-native-community/cli info

System:
  OS: macOS 27.0.1
IDEs:
  Xcode: 27.0/27A266a
npmPackages:
  react: 19.2.3
  react-native: 0.87.1
iOS:
  hermesEnabled: true
  newArchEnabled: true

0.88 was checked on the same machine with react-native@0.88.0-rc.4, on the iOS Simulator (iPhone 17 Pro, iOS 26.5).

Stacktrace or Logs

// 0.87.1's guard: the detached anchor is corrected
[mvcp] adjust guardMode=0 offset=2000.0 contentH=22773.0 anchor=alive tag=6320/6320 attached=NO prevY=1851.0 newY=4251.0
[mvcp] adjust corrected deltaY=+2400.0 offset 2000.0 -> 4400.0
// main's guard
[mvcp] adjust guardMode=1 offset=2000.0 contentH=22773.0 anchor=alive tag=7212/7212 attached=NO prevY=1851.0 newY=4251.0
[mvcp] adjust bail: anchor not a subview of the content view (main)

The full logs are in the repro's evidence/.

MANDATORY Reproducer

https://github.com/mozzius/scrollview-mvcp-anchor-repro

Screenshots and Videos

• ios-clipped-anchor-0.88.0-rc.4.mp4: unmodified 0.88.0-rc.4.

• ios-clipped-anchor.mp4: the 0.87.1 repro, switching between the guards.