I would like to determine if a rect inside a window is completly visible.
I have found RectVisible, but that function determines if any part of the rect is visible, I want to know if the entire rect is visible.
Is there any function for this?
First get the system clipping region (the visible region of a window) into a region by using GetRandomRgn. Read more about the 'system region' here. Then, offset that region since it is in screen coordinates (the article I linked has an example). After that, create a region from your rectangle with CreateRectRgn and combine the parts of your 'rectangle region' with those that are not part of the 'system region': that is calling CombineRgn passing the rectangle region as the first region, and the system region as the second region, and RGN_DIFF as the fnCombineMode. If the result is NULLREGION then your rectangle is fully visible - it is not fully or partially covered by any window (top level or not), or it is not fully or partially off-screen.
All in all, there's a probability that you're approaching your problem the wrong way around. If you've told what you've been trying to achieve someone could probably suggest a simpler approach.