Last FlatList item component re-mounted when adding new elements at the beginning of the list

[exploIF]

Description

Each time a new component is added at the beginning or in the middle of the list, the last component (the one that was added first) re-mounts. I've noticed that when a new component is added to the list, a spacer component is created and it always replaces last component in that list, even if new components are added at the beginning or in the middle of the list. This makes last item to completely re-mount. This problem only occurs when the initialNumToRender is exceeded.

Steps to reproduce

1. Run repro from linked snack.
2. Press add button multiple times (at least 4).
3. You will see in the terminal that after reaching initialNumToRender last item is unmounted.

React Native Version

0.74.1

Affected Platforms

Runtime - Android, Runtime - iOS

Output of npx react-native info

System:
  OS: macOS 14.4.1
  CPU: (11) arm64 Apple M3 Pro
  Memory: 110.58 MB / 18.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 21.7.3
    path: /opt/homebrew/bin/node
  Yarn:
    version: 4.1.1
    path: /opt/homebrew/bin/yarn
  npm:
    version: 10.5.0
    path: /opt/homebrew/bin/npm
  Watchman:
    version: 2024.04.08.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.14.3
    path: /opt/homebrew/bin/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 23.5
      - iOS 17.5
      - macOS 14.5
      - tvOS 17.5
      - visionOS 1.2
      - watchOS 10.5
  Android SDK:
    API Levels:
      - "31"
      - "34"
    Build Tools:
      - 30.0.3
      - 33.0.1
      - 34.0.0
    System Images:
      - android-34 | Google APIs ARM 64 v8a
      - android-TiramisuPrivacySandbox | Google Play ARM 64 v8a
    Android NDK: Not Found
IDEs:
  Android Studio: 2023.2 AI-232.10300.40.2321.11668458
  Xcode:
    version: 15.4/15F31d
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.10
    path: /usr/bin/javac
  Ruby:
    version: 2.6.10
    path: /usr/bin/ruby
npmPackages:
  "@react-native-community/cli": Not Found
  react:
    installed: 18.2.0
    wanted: 18.2.0
  react-native:
    installed: 0.74.0
    wanted: 0.74.0
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: false
iOS:
  hermesEnabled: true
  newArchEnabled: false

Stacktrace or Logs

-

Reproducer

https://snack.expo.dev/@exploifswm/flat-list-item-re-mount

Screenshots and Videos

No response

[deepanshushuklad11]

@cortinico Any update on this ?

[cortinico]

@cortinico Any update on this ?

Nope no updates. We're hyperfocused on fixing New Architecture bugs at the moment, so unless this is a New Architecture specific bug, we won't be able to fix it in the immediate future

[deepanshushuklad11]

@cortinico This is happening in both new arch and old .

[cortinico]

@cortinico This is happening in both new arch and old .

Yup but is not New Architecture specific (i.e. is not a regression)

[deepanshushuklad11]

@cortinico anyways i started debugging. I think the issue is here https://github.com/facebook/react-native/blob/8c8c77b974ce3d4c3036361e56276a6ff2b02208/packages/virtualized-lists/Lists/VirtualizedList.js#L716 This check is not needed in my opinion , but currently i am not sure of its impact . If you could help me guiding in right direction that would be of great help

[deepanshushuklad11]

@exploIF can you try this out .