All is describe here : http://forum.openlaszlo.org/showthread.php?p=28402
Including a handy example to reproduce the bug and a "bad" trick to avoid it

Quoted from the thread :
I'm implementing drag and drop functionality but have found that overlapping views do not always receive the onmousetrackxxxxxx events. Please see the simple case below.

1. When the windows are separate, then dragging the green view causes each window to fire the onmousetrackover event. (Great).

2. When the windows are resized etc and the blue window is dragged inside the red window, both windows still fire the onmousetrackover event. (Great).

3. When the windows are resized etc and the red window is dragged inside the blue window, only the blue windows fires the onmousetrackover event. (Boo). A related effect of this is that if you try to drop something into the red window it is in fact the blue window the receives the drop.