Fix onStartReached not called

[janicduplessis]

Summary:

I've seen some cases where onStartReached calls would be missed. This seems to happen because we bail out of calling onStartReached if our current viewport is not currently at index 0 (

react-native/packages/virtualized-lists/Lists/VirtualizedList.js

Line 1553 in 7bc9244

this.state.cellsAroundViewport.first === 0 &&

). However if no further scroll event happens after finally adjusting the viewport to render 0 we never end up calling onStartReached. To fix this we can try calling _maybeCallOnEdgeReached whenever the viewport changes.

I also seen cases where firstVisibleItemKey would be incorrect because of the early return here. To be honest I am not exactly sure why, but prevState.renderMask.numCells() doesn't actually have the item count from the previous render, but the current item could, which would cause that code to return incorrectly and not update firstVisibleItemKey. To fix this I am also tracking lastItemCount in the state and comparing this too instead of relying only on prevState.renderMask.numCells().

Changelog:

[GENERAL] [FIXED] - Fix onStartReached not called in some cases

Test Plan:

Tested in the Expensify app to test that this fixes some cases we saw of onStartReached not being called.