The drag source should be provided with mouse coordinates so that the
drag gesture can be animated. For Swing, we would like to provide an
alpha blended image that follows the mouse pointer as a display of
drag feedback. The drag source is currently not provided mouse coordinates
to make this possible.

The DragSourceListener.dragOver() method might be used if the mouse
coordinates could be fetched from the DragSourceDragEvent. A problem
with that method however is that it is documented as being conditionally
called. For drag feedback all mouse motion is needed.