I found this while investigating QTBUG-49876: when an external (inter-process) drag which was started from a QML MouseArea finishes, the MouseArea doesn't get the mouse release event and thinks that the drag is still active. The target item is being dragged around, even though no mouse buttons are pressed anylonger.

I don't know why, but for some reason the QSimpleDrag::eventFilter() method is being called with the source object parameter set to the QShapedPixmapWindow, and forwarding the mouse release event to this window won't have any effect.