I am drawing a rectangle in a active shell of eclipse with white color;

and after some event(Keyboard or mouse) I want to remove that rectangle but I am unable to do so .
I am disposing GC , however it shows that rectangle but When I minimize eclipse shell(In which rectangle is drawn) and maximize after that . It doesnt show while area as I wanted. But I want it immediate after event not after minimizing and then maximizing window.

Thanks in advance...