private void sendMessageToClientArea( HWND hwnd, int uMsg, LPARAM lParam ) {
			// get mouse x/y in window coordinates
			LRESULT xy = screen2windowCoordinates( hwnd, lParam );

			// send message
			User32.INSTANCE.SendMessage( hwnd, uMsg, new WPARAM(), new LPARAM( xy.longValue() ) );
		}
Same implementation as RGB(r,g,b) macro in wingdi.h.
flatlaf-natives/flatlaf-natives-jna/src/main/java/com/formdev/flatlaf/natives/jna/windows/FlatWindowsNativeWindowBorder.java