GetClipRgn not sufficient

[newdrug]

I'm searching a way to get the visible region (HRGN) of a window. GetClipBox gives only a bounding box:

"The GetClipBox function retrieves the dimensions of the tightest bounding rectangle that can be drawn around the current visible area on the device. The visible area is defined by the current clipping region or clip path, as well as any overlapping windows."

I tried GetClipRgn, but it only checks application set clipping regions and paths and therefor returns NULL in most cases. In contrast to GetClipBox, GetClipRgn doesn't take care of overlapping windows or parts of a window that are out of the screen bounds. I would like to have the really visible region of a specific window.

Thank you in advance.