When scrolling up through a list then Listview::indexAt() may return -1 for an item that is visible. If you are calling indexAt() in response to onContentYChanged then sometimes it will return -1 when scrolling upwards, even though when scrolling downwards it doesn't have this problem.

Attached example reproduces this problem, just scroll down the list and then scroll up, the debug output will show what indexAt is returning.