CWindowDC dc(this);
CRgn rgn;
CRect rcWindow, rcClient;
GetWindowRect(rcWindow);
GetClientRect(rcClient);
ClientToScreen(rcClient);
rcClient.OffsetRect(-rcWindow.TopLeft());
rgn.CreateRectRgn(rcClient.left, rcClient.top, rcClient.right, rcClient.bottom);
dc.SelectClipRgn(&rgn, RGN_DIFF);
