title: Transparent non-client area
body: <p>I am creating a TextBox control subclass using Windows Forms. I am using the WM_NCCALCSIZE to change the client area (to enlarge the non-client area) but I have not found any solution on how to make the non-client area transparent. 
  Is there any simple way to paint to non-client area in WM_NCPAINT in transparent way to duplicate the underlying window's background in intersecting region of the child control?</p>