Call this member function to get the window region of a window.
The window region determines the area within the window where the operating system permits drawing. The operating system does not display any portion of a window that lies outside of the window region.
The coordinates of a window's window region are relative to the upper-left corner of the window, not the client area of the window.
To set the window region of a window, call CWnd::SetWindowRgn.