void add_with_viewport(GtkWidget child);

Used to add children without native scrolling capabilities. This is simply a convenience function; it is equivalent to adding the unscrollable child to a viewport, then adding the viewport to the scrolled window. If a child has native scrolling, use add() instead of this function.

The viewport scrolls the child by moving its GdkWindow, and takes the size of the child to be the size of its toplevel GdkWindow. This will be very wrong for most widgets that support native scrolling; for example, if you add a widget such as GtkTreeView with a viewport, the whole widget will scroll, including the column headings. Thus, widgets with native scrolling support should not be used with the GtkViewport proxy.