Jitter/flickering when scrolling upwards through variable-height items after prepending data

Description

When items are prepended to a FlashList with variable-height content, scrolling upwards through the newly added items causes visible jitter/flickering. The content visibly jumps as items are measured during scroll.

Current behavior

• Fixed-height items: No jitter
• Variable-height items: Jitter present
• FlatList does NOT exhibit this issue on iOS

FlashList (jitter)

https://github.com/user-attachments/assets/77ef19fc-7b7b-424a-8125-fd6408a931cb

FlatList (no jitter)

https://github.com/user-attachments/assets/2aeedde9-5fa0-439b-8080-9c51d7a2fc64

Expected behavior

Smooth scrolling without significant jitter when scrolling upwards through prepended variable-height items.

Reproduction

Expo Snack or minimal reproduction link:

https://snack.expo.dev/@jessejanderson/flash-list-jitter

Platform

• iOS
• Android
• Web (if applicable)

Environment

FlashList version: 2.2.0
React Native version: 0.81 (Expo SDK 54)

Additional context

In the app where I first ran into this issue, I've tried a number of recommendations from the v2 documentation and other sources, but none of them have resolved the issue.

• Memoized props
• Proper keyExtractor usage to return unique, stable keys for each item.
• Using getItemType to better predict height for different item types.
• Tested various drawDistance values. Nothing but extremely high values (10000+) made a significant difference.
• Deferred state updates during scroll.
• scrollEventThrottle={16}
• Confirmed no key props
• A bunch of random other things. 🙃

None of these resolved the issue (other than the extremely high drawDistance values, which eventually break when the prepended/new item count is high enough).

Checklist

• I've searched existing issues and couldn't find a duplicate
  (there are some similar issues, but involve multi-column layouts or other more complex setups)
• I've provided a minimal reproduction (Expo Snack preferred)
• I'm using the latest version of @shopify/flash-list
• I've included all required information above

Start with the linked Expo Snack reproduction and run it on iOS with FlashList 2.2.0 and React Native 0.81. Compare prepended fixed-height and variable-height items while scrolling upward, using the FlatList recording as a baseline. Trace the measurement and prepend behavior responsible for visible jumps, and verify that variable-height prepended items scroll smoothly without significant jitter.