VirtualizedSectionList passes incorrect leading/trailingItem to ItemSeparatorComponent

Description

Our app has a <SectionList> which was intermittently rendering ItemSeparatorComponents with wildly incorrect leadingItem/trailingItem props, including undefined, which as far as I can tell should never happen.

(While I’m here, I would also like to complain that the type definition for this prop is React.ComponentType<any> so I also spent a bit of time trying to work out if undefined was even an expected value.)

After investigation, it turns out that ItemWithSeparator keeps these values in state, so when the same cell happens to re-render with new props (for example when the virtualized list renders a different region), the separator keeps the previous leadingItem/trailingItem, which can be undefined if that previous row was at a section boundary.

Steps to reproduce

Have a <SectionList ItemSeparatorComponent={...}>, scroll around a bit, and observe the component is getting the wrong items.

Or, I have created a convenient failing unit test here: https://github.com/facebook/react-native/pull/55707

React Native Version

0.84.0

Affected Platforms

Runtime - Android, Runtime - iOS, Runtime - Web, Runtime - Desktop

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

~/src/vend/react-native$ npx @react-native-community/cli info
info Fetching system and libraries information...
System:
  OS: macOS 15.7.4
  CPU: (8) arm64 Apple M3
  Memory: 133.81 MB / 16.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 20.19.5
    path: ~/.asdf/installs/nodejs/20.19.5/bin/node
  Yarn:
    version: 1.22.22
    path: ~/.asdf/installs/nodejs/20.19.5/bin/yarn
  npm:
    version: 10.8.2
    path: ~/.asdf/plugins/nodejs/shims/npm
  Watchman:
    version: 2025.09.08.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.15.2
    path: /Users/wolf/.asdf/shims/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 25.1
      - iOS 26.1
      - macOS 26.1
      - tvOS 26.1
      - visionOS 26.1
      - watchOS 26.1
  Android SDK: Not Found
IDEs:
  Android Studio: 2025.2 AI-252.25557.131.2521.14432022
  Xcode:
    version: 26.1.1/17B100
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.16
    path: /usr/bin/javac
  Ruby:
    version: 2.7.6
    path: /Users/wolf/.asdf/shims/ruby
npmPackages:
  "@react-native-community/cli": Not Found
  react:
    installed: 19.1.0
    wanted: 19.2.3
  react-native: Not Found
  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

● VirtualizedSectionList › syncs ItemWithSeparator separator props when list re-renders with new leadingItem/trailingItem

    expect(received).toEqual(expected) // deep equality

    Expected: ArrayContaining [{"leadingItem": "b", "trailingItem": "a"}]
    Received: [{"leadingItem": "b", "trailingItem": "c"}, {"leadingItem": "a", "trailingItem": "b"}]

      254 |     // state from when "b" was at index 1 (leadingItem: a, trailingItem: c), so
      255 |     // the separator receives stale props and this assertion fails.
    > 256 |     expect(separatorPropsReceived).toEqual(
          |                                    ^
      257 |       expect.arrayContaining([
      258 |         {leadingItem: 'b', trailingItem: 'a'},
      259 |       ]),

      at Object.toEqual (packages/virtualized-lists/Lists/__tests__/VirtualizedSectionList-test.js:256:36)
      at asyncGeneratorStep (packages/virtualized-lists/Lists/__tests__/VirtualizedSectionList-test.js:15:37)
      at asyncGeneratorStep (packages/virtualized-lists/Lists/__tests__/VirtualizedSectionList-test.js:15:37)

MANDATORY Reproducer

https://github.com/facebook/react-native/pull/55707

(This includes both a failing unit test for the underlying problem and an RNTester Playground reproducer with SectionList as an example.)

Screenshots and Videos

N/A

Inizia con ItemWithSeparator e il test non riuscito in packages/virtualized-lists/Lists/tests/VirtualizedSectionList-test.js; esegui quel test per riprodurre le props obsolete del separatore. Il lavoro è completato quando il test passa e ItemSeparatorComponent riceve i valori correnti di leadingItem e trailingItem dopo che la lista esegue il rendering di una regione diversa.