> From what I can tell up to and including Windows XP the painting was done in software, i.e. the CPU was processing all the pixels of the window. I also believe that the frame and the client area were painted into the same pixel buffer that would eventually be copied to the main screen buffer.

On non-compositing windowing systems each window is designated a region of the front buffer to draw directly to it. Drawing outside that region was unrestricted and easy.

The rounded corners in Windows XP weren't hard-coded anywhere, they were just transparent parts of the bitmaps used in the theme engine. Even though they were transparent, they behaved like the opaque parts of the window border.