fix(hooks): eliminate scroll jitter when prepending items at bottom

[Hector-Zhuang]

Description

Fixes #2094

This pull request addresses the jittery scroll animation on iOS when items are prepended to the top of the list while the list is positioned at the bottom.

The Problem

When items are added to the top, the LayoutManager immediately calculates a new scroll offset to "lock" the current visible messages in place so they don't jump.

Simultaneously, because animateAutoScrollToBottom is enabled and the list is at the bottom, the component triggers a scrollToEnd({ animated: true }).

The Fix

I have modified useBoundDetection to detect when the list is already anchored at the bottom (via isNearBottom.current).

• When new data is added at the top (prepend), the PR ensures that the scroll synchronization happens silently (animated: false) to avoid clashing with the coordinate recomputation.
• Smooth animations are preserved for regular "bottom-up" updates (append) where no coordinate shift conflict exists.

Reviewers’ hat-rack 🎩

• isNearBottom.current reliability: Verify this ref accurately tracks the bottom state during rapid data updates.
• Prepend vs Append: Ensure that only top-insertions skip the animation, while new incoming messages at the bottom still animate smoothly.
• Test with the provided Expo Snack: https://snack.expo.dev/@hectorton/cranky-red-graham-crackers
• Verify no regression in standard Vertical/Horizontal lists without maintainVisibleContentPosition.

Screenshots or videos (if needed)

Reproduction Video (from #2094):
https://github.com/user-attachments/assets/11cc145c-f65b-4e34-8585-961a455a27bc

Fixed Behavior:
The list now remain stable when items are added to the top, and smoothly scrolls only when new items are added to the bottom.

[naqvitalha]

Thanks for the PR!

There's actually an existing signal that might fit this use case perfectly: recyclerViewManager.ignoreScrollEvents. It's set to true for ~100ms during offset corrections (i.e., when a prepend triggers a scroll adjustment), which is exactly the window where the animation conflict happens.

So instead of the new ref, this should work as a one-line change in runAutoScrollToBottomCheck:

scrollViewRef.current?.scrollToEnd({
  animated: shouldAnimate && !recyclerViewManager.ignoreScrollEvents,
});

This way animation is only skipped when an offset correction is in progress, and normal appends still animate smoothly. Could you try this out and see if it fixes the jitter in your repro?

[naqvitalha]

Also please sign the CLA

[Hector-Zhuang]

Thanks. This fix also works. i've updated this branch.

[naqvitalha]

Looks good. If you can sign the CLA we can merge it.

[Hector-Zhuang]

Just signed the CLA. Thx!