Fix dragdrop.js when drop area contains scrollbars.

Fix dragdrop.js when drop area contains scrollbars.

Mouseover/mouseout events may not trigger when moving over a scrollbar,
so we need to observe the current position by looking at the mousemove
event instead.  Works out for the better since we are already processing
the mousemove event - allows us to remove 2 more global event observe
calls.