display update problem

Hi,

Does anybody know why there is a problem that a 3D display in a JApplet
somestimes won't update a area which was
sheltered from the other window, and then moved out, it kept "gray" until I
clicked on the display pane for enforcing
its repaint action.

This strange beahavior made me crazy!!!
Please help me figure this out!!

Thanks a lot,

Tim