Call this member function to set a window's region.
A handle to a region.
If TRUE, the operating system redraws the window after setting the region; otherwise, it does not. Typically, set bRedraw to TRUE if the window is visible. If set to TRUE, the system sends the WM_WINDOWPOSCHANGING and WM_WINDOWPOSCHANGED messages to the window.
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.
After a successful call to SetWindowRgn, the operating system owns the region specified by the region handle hRgn. The operating system does not make a copy of the region, so do not make any further function calls with this region handle, and do not close this region handle.