https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=101102

If you have a GtkViewport with some insensitive widgets inside
of it, then when the mouse is over an insensitive widget, 
you can't scroll the viewport with the scrollwheel.

The problem is that the GDK_SCROLL events are just getting
eaten rather than being propated up.

From gtk_propagate_event()

          handled_event = !GTK_WIDGET_IS_SENSITIVE (widget) ||
gtk_widget_event (widget, event);

The only way I can think if fixing this is to say that scroll
events are special and do propagate up from insensitive widgets
while other events don't

 if (GTK_WIDGET_IS_SENSITIVE (widget))
   handled_event = event->type != GDK_SCROLL;
 else
   handled_event = gtk_widget_event (widget, event)

Ugh.