dragLeaveEvent() not always called

This bug can be repeated with the "Drop Site" demo application on a Mac, using a Cocoa build of Qt.

The Carbon version works, but the bug has also been observed on Linux.

To repeat the bug, drag a file from the Finder into the <drop content> area, then (without dropping) pull the cursor out of the area quickly. You will see that sometimes the <drop content> area remains highlighted (as if the drag is still in progress).

The reason is dragLeaveEvent() was not called.

The bug is easier to repeat when a slight delay (e.g. usleep(100)) is placed in the dragMoveEvent() call.