Updating viewport position from within the application?

[Toralf Lund <toralf procaptura com>]

What exactly do I have to do in order to set the current "scroll 
position" of a viewport from within (i.e. with C++ calls as opposed to 
GUI interaction) in Gtk 2?

I've tried

viewscroll->get_vadjustment()->set_value(value)


where viewscroll is the Gtk::Viewport, and value the new position 
(obviously.) This updates the slider as expected, but the data isn't 
actually scrolled (so the portion visible is different from what the 
slider indicates.)

- Toralf