I am not sure whether this is a bug or not.
document.elementFromPoint() currently does not take page scroll into consideration.
If the input parameter (x, y) is the point in viewport, it should be increased by (document.body.scrollLeft, document.body.scrollTop), then apply zoomFactor, in order to get the correct point in the document.
Or it is expected user to pass in (x, y) with scoll offset, such as
(event.clientX + document.body.scrollLeft, event.clientY + document.body.scrollTop)?
Can you gather stats on what the other browsers do?
My experiments show that Firefox behaves as the spec describes, and I don't believe IE implements this API. We should probably change.
I take it back, IE implements it, but I don't have a windows machine to test on at the moment.
Created attachment 39511 [details]
Comment on attachment 39511 [details]
Fixed in r48334.
This broke dragging files on to file inputs: bug 29276.