The container calls this function to get the window handle of the control.
HRESULT GetWindow( HWND* phwnd );
See IOleWindow::GetWindow in the Platform SDK.
Some containers will not work with a control that has been windowless, even if it is currently windowed. In ATL's implementation, if the CComControl::m_bWasOnceWindowless data member is TRUE, the function returns E_FAIL. Otherwise, if *phwnd is not NULL, GetWindow assigns phwnd to the control class's data member m_hWnd and returns S_OK.