Flatlist exposes onViewableItemsChanged - which can be used to determine which elements are currently
visible within a list. This works correctly.
If a ListHeaderComponent is also given, the calculations used for determining which items are in view
will be incorrect - by exactly the height of the header.
in Libraries/Lists/VirtualizedList.js, this._headerLength is initialised as 0 and then updated on render
to store the length of the header dynamically, but then this value is not used anywhere else.
We found the easiest way was to subtract the this._headerLength from the offset calculation, such as
_selectOffset(metrics: {x: number, y: number}): number {
- return !this.props.horizontal ? metrics.y : metrics.x;
+ return (!this.props.horizontal ? metrics.y : metrics.x) - this._headerLength;
}