The scrolled window can work in two ways. Some widgets have native scrolling support; these widgets have "slots" for Gtk_Adjustment objects. The scrolled window installs Gtk_Adjustment objects in the child window's slots using the "set_scroll_adjustments" signal (Conceptually, these widgets implement a "Scrollable" interface).
The second way to use the scrolled window is useful with widgets that lack the "set_scroll_adjustments" signal. The Gtk_Viewport widget acts as a proxy, implementing scrollability for child widgets that lack their own scrolling capabilities.
If a widget has native scrolling abilities, it can be added to the Gtk_Scrolled_Window with Gtk.Container.Add. If a widget does not, you must first add the widget to a Gtk_Viewport, then add the Gtk_Viewport to the scrolled window. The convenience function Add_With_Viewport does exactly this, so you can ignore the presence of the viewport.
If you want to create your own new widget type that can be inserted directly into a scrolled_window, you need to specify a signal for Set_Scroll_Adjustments in the call to Gtk.Object.Initialize_Class_Record.
The viewport scrolls the child by moving its Gdk_Window, and takes the size of the child to be the size of its toplevel Gdk_Window. This will be very wrong for most widgets that support native scrolling; for example, if you add a Gtk_Clist with a viewport, the whole widget will scroll, including the column headings. Thus Gtk_Clist supports scrolling already, and should not be used with the GtkViewport proxy.
A widget supports scrolling natively if it contains a valid "set_scroll_adjustments" signal.