[Android] One-frame jump on prepend with maintainVisibleContentPosition

Description

On Android with New Architecture, a ScrollView with maintainVisibleContentPosition draws one stale frame when an item is prepended. The rows move down by the height of the new item for one frame, then move back. In a chat list with live traffic this happens on every incoming message

The bug needs two conditions, both can be encountered when utilising virtualised list (observed first with LegendList 3):

1. The anchor child and the item children move in the same Fabric mount, so the helper corrects the scroll offset for that mount
2. Nothing in that mount changes the size of the content container. In LegendList the container height is an Animated.Value, so it reaches native in a separate update. If the content resizes in the same mount, the layout pass invalidates the ScrollView and the frame is correct. That is why a plain prepend of flex children does not show it

The stale frame comes from MaintainVisibleScrollPositionHelper. In didMountItems it calls scrollToPreservingMomentum on the ReactScrollView, but nothing invalidates the ScrollView in that frame. View.scrollTo only calls postInvalidateOnAnimation, which lands in the next frame. The moved children have already invalidated themselves for the current frame, so the current frame draws the new child positions through the old ScrollView display list, which still holds translate(-oldScrollY)

Steps to reproduce

1. Clone Reproduction repo and run yarn && yarn android (yarn berry)
2. Watch the list
3. Tap invalidate ScrollView after mount to see the jump stop

React Native Version

0.86.3

Affected Platforms

Runtime - Android

Areas

Fabric - The New Renderer

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

System:
  OS: macOS 26.6.2
  CPU: (14) arm64 Apple M4 Pro
  Memory: 225.94 MB / 24.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 20.19.6
    path: /Users/szymon/.local/share/mise/installs/node/20.19.6/bin/node
  Yarn:
    version: 4.18.0
    path: /Users/szymon/.local/share/mise/installs/node/20.19.6/bin/yarn
  npm:
    version: 10.8.2
    path: /Users/szymon/.local/share/mise/installs/node/20.19.6/bin/npm
  Watchman:
    version: 2026.07.27.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.17.0
    path: /opt/homebrew/bin/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 25.2
      - iOS 26.2
      - macOS 26.2
      - tvOS 26.2
      - visionOS 26.2
      - watchOS 26.2
  Android SDK:
    API Levels:
      - "31"
      - "35"
      - "36"
    Build Tools:
      - 35.0.0
      - 36.0.0
      - 36.1.0
    System Images:
      - android-35 | Google APIs ARM 64 v8a
      - android-35 | Google Play ARM 64 v8a
      - android-35 | Pre-Release 16 KB Page Size Google APIs ARM 64 v8a
      - android-36.1 | Pre-Release 16 KB Page Size Google APIs ARM 64 v8a
    Android NDK: Not Found
IDEs:
  Android Studio: 2025.3 AI-253.30387.90.2532.14935130
  Xcode:
    version: 26.3/17C529
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.20
    path: /usr/bin/javac
  Ruby:
    version: 3.4.9
    path: /Users/szymon/.local/share/mise/installs/ruby/3.4.9/bin/ruby
npmPackages:
  "@react-native-community/cli": Not Found
  react:
    installed: 19.2.3
    wanted: 19.2.3
  react-native:
    installed: 0.86.3
    wanted: 0.86.3
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: Not found
  newArchEnabled: Not found
iOS:
  hermesEnabled: Not found
  newArchEnabled: Not found

Stacktrace or Logs

Do not apply

MANDATORY Reproducer

https://github.com/Critteros/rn-android-mvcp-one-frame-jump

Screenshots and Videos

Reproduction:

https://github.com/user-attachments/assets/8e3991da-a158-4377-a240-2063e9722c8e

With invalidation:

https://github.com/user-attachments/assets/4731c14c-cf92-463f-b6c2-2e8917145aa8

Start by running the mandatory reproduction repo and reading MaintainVisibleScrollPositionHelper.didMountItems together with ReactScrollView.scrollToPreservingMomentum. Trace why the scroll view is not invalidated in the same frame as the mount. Done means the one-frame jump no longer appears when prepending items under the stated conditions, including the reproduction case.