The SetWindowRgn function sets the window region of a window. The window region determines the area within the window where the system permits drawing. The system does not display any portion of a window that lies outside of the window region
int SetWindowRgn(
HWND hWnd, // handle to window whose window region is to be set
HRGN hRgn, // handle to region
BOOL bRedraw // window redraw flag
);
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