GtkNotebookWindowCreationFunc is only called if you drop a tab on the root window. Some desktop environments, eg ROX, appear to completely cover the root window with their own backdrop, and in other situations it may be difficult to find a clear bit of root window anyway. This limits the usefulness of the hook, and is also inconsistent with gnome-terminal, which allows a new window to be created by dragging a tab to anywhere that isn't a gnome-terminal window. I think there should be an additional function to enable the hook to be called wherever the tab is dropped (other than a notebook with the same group id).