Part of ENG-3976
Fetches more messages for threads, when the user is near the end of the thread list.
Additionaly if there not enough threads to fill the whole thread list, onItemsRendered doesn't fire (quirk of react-window) so we wouldn't fetch more messages. Because of that there's an useEffect that calculates if the current thread list is smaller than the total height and if yes tries fetching more messages.

Depends on D8234

Try scrolling the thread list and check if threads are correctly loading and there are no jumping threads.