Add experimentalMaintainTopContentPosition support to FlashList

[friyiajr]

Description

Fixes (#547)

This pull request adds experimental support for a prop called experimentalMaintainTopContentPosition. This prop will maintain the position of the list when items are added from the top down. Unlike maintainVisibleContentPosition in React-Native we don't allow the user to specify a target for where they want the anchor to be. We always specify it as the top.

Reviewers’ hat-rack 🎩

Run tests in release mode. In debug mode there can be some flashing from clearGaps

• Make sure nothing regressed in how FlashList worked before
• Turn on experimentalMaintainTopContentPosition, add a keyExtractor and make sure you can maintain the position of the list when adding items to the front of the array
• Make sure this works Vertically and Horizontally

Here is an example of the playground I was using for testing List.tsx
ListDebugPlayground.txt

Screenshots or videos (if needed)

The app working Vertically:

Simulator.Screen.Recording.-.iPhone.14.Pro.-.2023-02-25.at.11.15.58.mp4

The app working Horizontally:

Simulator.Screen.Recording.-.iPhone.14.Pro.-.2023-02-25.at.11.19.40.mp4

Checklist

• I have added a changelog entry following the Keep a Changelog guidelines.

[arthurgeron-work]

Great work! This is exactly what I need. I don't see any android code changes, is this iOS only prop?

[friyiajr]

@arthurgeron-work for now yes it is. We are currently working on Android

[batuhansahan]

Great work! Does this work with inverted={true}

[zulfio]

and this is what i need, thanks for creating this PR man

[geo-vi]

ETA for when this will be merged?

Awesome PR!

[friyiajr]

Closed in favor of this PR which addresses iOS and Android #824