Viv,

If there is another window on top then your drawing is clipped to the window context not to anything else. You only draw to the context/surface associated with the window.

I am not advocating going to the display buffer directly as you can with DirectDraw or DirectX surfaces.

If you take my example of the tooltip then if I draw a "hot-area" from the MouseMove event I do not draw directly onto the tooltip - it does not get corrupted.

Regards
Neil