Why does gtk_widget_shape_combine_region only work for toplevel windows?

[<feihu929 sina com>]

I am trying to make some parts of the widget (more specific WebKitGTK) 
invisible both for clicks and on screen. I found the function:

void gtk_widget_shape_combine_region (GtkWidget *widget,
                             cairo_region_t *region);


  Sets a shape for this widget’s GDK window. This allows for transparent
  windows etc., see gdk_window_shape_combine_region() for more
  information.


It is exactly what I want, but it doesn't work for the single 
widgets, but it works if I call it on the toplevel window. 
gdk_window_shape_combine_region() in its description says: 


  This function works on both toplevel and child windows.


I'm not sure what do they mean by child windows, but I thought it 
could be widgets, because as I've read GTK doesn't support child 
windows.

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I found use gtk_widget_shape_combine_mask in GTK2, it can implement your
 case, but gtk_widget_shape_combine_region can not in GTK3