onViewableItemsChanged isn't correct without specifying the header height in getItemLayout

Description

I am unsure if this is a bug or expected behaviour. If it's the latter then could be worth adding to the docs.
Also I can only test on my version (0.73.8) unfortunately. But I can't find any issues that suggest it's been fixed recently, so hopefully a useful issue for people. At the very least I couldn't find anything about this online so maybe useful for others.

If I have a simple Flatlist with a header like this:

 <FlatList
            data={Array.from({ length: 20 })}
            renderItem={({ index }) => (
                <V height={index === 1 ? 250 : 150} p={20} dbg>
                    <T>{index}</T>
                </V>
            )}
            getItemLayout={(data, index) => {
                if (index === 0) {
                    return { length: 150, offset: 200, index }
                }
                if (index === 1) {
                    return { length: 250, offset: 150 + 200, index }
                }
                return { length: 150, offset: 150 * (index - 2) + 200 + 150 + 200, index }
            }}
            ListHeaderComponent={<V height={200} width="100%" />}
            viewabilityConfig={viewabilityConfig}
            onViewableItemsChanged={({ viewableItems }) => {
                console.log('>>>', viewableItems, viewableItems.length)
            }}
        />

If I remove the header height (200) from the getItemLayout offset calculation, onViewableItemsChanged fires incorrectly.

If I remove getItemLayout entirely, the logic works fine regardless.

I would have thought by the time the layout has rendered, the onViewableItemsChanged logic would be able to use the actual layout value vs relying on getItemLayout? It would certainly make things robust for us since the header height is quite dependent on various things.

Steps to reproduce

Run the code snippet above. Scroll down and observe that onViewableItemsChanged fires correctly. Remove 200 from getItemLayout calculation and observe it is now incorrect.

React Native Version

0.73.8

Affected Platforms

Runtime - Android, Runtime - iOS

Output of npx react-native info

System:
  OS: macOS 14.4
  CPU: (8) x64 Apple M1 Pro
  Memory: 29.45 MB / 16.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 18.19.1
    path: ~/.nvm/versions/node/v18.19.1/bin/node
  Yarn:
    version: 1.17.3
    path: ~/.yarn/bin/yarn
  npm:
    version: 10.2.4
    path: ~/.nvm/versions/node/v18.19.1/bin/npm
  Watchman:
    version: 2024.12.02.00
    path: /usr/local/bin/watchman
Managers:
  CocoaPods:
    version: 1.15.2
    path: /Users/mattdalton/.rvm/gems/ruby-2.7.7/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: Not Found
IDEs:
  Android Studio: 2021.2 AI-212.5712.43.2112.8609683
  Xcode:
    version: 15.4/15F31d
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.10
    path: /usr/bin/javac
  Ruby:
    version: 2.7.7
    path: /Users/mattdalton/.rvm/rubies/ruby-2.7.7/bin/ruby
npmPackages:
  "@react-native-community/cli": Not Found
  react:
    installed: 18.2.0
    wanted: 18.2.0
  react-native:
    installed: 0.73.8
    wanted: ^0.73.8
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: false
iOS:
  hermesEnabled: true
  newArchEnabled: false

Stacktrace or Logs

N/A

Reproducer

https://github.com/matt-dalton/flatlistviewable

Screenshots and Videos

No response

Start with the linked flatlistviewable reproducer and its FlatList entry point, using React Native 0.73.8. Compare onViewableItemsChanged behavior with and without getItemLayout and the 200px ListHeaderComponent offset on Android and iOS. Done means determining whether the behavior is a bug or expected and adding a regression test or documenting the required offset.