Fix invalid onScroll events breaking FlatList

[lenaic]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

RCTScrollView was submitting onScroll events after an onContentSizeChanged event. This was caused by the setFrame call leading to a DocumentViewBoundsChanged notification that would get triggered a second time after the horizontal/vertical scrollers would be enabled/disabled.

This double notification would trigger a scrollViewDidScroll call with an onScroll event being sent because the scroll position would toggle temporarily due to the scrollers being shown/hidden.

This PR updates the scroll view to compare the scroll position with the last sent position after the scrollers were set. This fixes the wrong onScroll event during the setFrame because the scroll position is stable and valid by the time we do the comparison.

Changelog

[macOS] [FIXED] - Update ScrollView to emit only valid onScroll events

Test Plan

Tested by running RNTester on macOS with paper and using the FlatList examples.

[Saadnajmi]

Not obvious from the description, but how did this break Flatlist? This is separate from the general "Flatlist is slow on macOS" issue, right?

[lenaic]

Not obvious from the description, but how did this break Flatlist?

We encountered issues in product code where dynamic content loading was implemented using a FlatList and tracking the scroll position. Because the scroll view was emitting scroll events while the view frame was still being set, the FlatList would receive an incorrect scroll event with a 0 height which was breaking the virtual rendering.

This is separate from the general "Flatlist is slow on macOS" issue, right?

It is a separate issue. Here it was happening at mount and resize time.

[Saadnajmi]

Not obvious from the description, but how did this break Flatlist?

We encountered issues in product code where dynamic content loading was implemented using a FlatList and tracking the scroll position. Because the scroll view was emitting scroll events while the view frame was still being set, the FlatList would receive an incorrect scroll event with a 0 height which was breaking the virtual rendering.

This is separate from the general "Flatlist is slow on macOS" issue, right?

It is a separate issue. Here it was happening at mount and resize time.

My bad for close/open.. finger slipped.
Are you also planning to bring this to the 0.71 branch? That would be nice :)

[lenaic]

Are you also planning to bring this to the 0.71 branch? That would be nice :)

Absolutely! I'll submit another bug fix PR and then bring both changes over to 0.71.