widgetAt / topLevelAt broken on MacOS (regression)

Run the attached sample.
Move the 1st window, to reveal the second window. Move this second window over window #1. Click in the middle of the top window (Window #2)... Q_ASSERT is triggered.

This is actually a bug of topLevelAt (but since widgetAt relies on topLevelAt, this is broken as well).
In other words, these functions are ignorant about the toppest window at the supplied position.

This problem did not exist in Qt 4.8
This problem does not exist under Windows

This bug causes lots of trouble...