Performant scroll restoration when changing data

[aweary]

At @discord we use use FlashList to render the list of channels in a server. When you switch servers, we want to update the channel list so that the selected channel in that server is in view; this means we need to call scrollToIndex to move the user to the right index.

The issue we often see is that this leads to some pretty unfortunate jank. It goes something like this:

1. User is on Server A where the currently selected channel is at index 5
   1a. We use initialScrollIndex to make sure it starts rendering at the right offset
2. User switches to Server B where the channel is at index 25
3. The data array changes with all the new channels
4. FlashList renders with the new data
5. In a useLayoutEffect we attempt to scroll to the new index with scrollToIndex

Often times this leads to FlashList doing more work than needed, where it renders the channels for the wrong offset and then we correct it by calling scrollToIndex in that layout effect. We see a few scenarios, depending on how timings shake out:

1. Switching servers takes longer because it has to render the new list items, scroll, and then render again
2. There is visible jank where FlashList appears to commit the changes for the new data and then the scroll updates the list

I'm wondering: is a more performant way to update the scroll offset when data changes? Ideally we could tell FlashList that when data changes, it should start rendering from a new scroll index; listening for changes to initialScrollIndex could do that, though it only seems to work for the initial mount and that would probably be a breaking change.