Scrollbar detection for full container

[Per Hermansson <hermansson per bredband net>]

Hi all

I have two GtkTables which are inside its own GtkViewport and 
GtkScrolledWindow.
Both tables are fixed size and dynamically populated with widgets. When 
adding widgets I want to
keep adding them to the first table just before the scrollbars appear, 
then I want to new widgets
to be added to the second one.
When the second one get full too I want to add new widgets to the first 
table and now see scrollbars.

My first attempt was checking 
GTK_WIDGET_VISIBLE(gtk_scrolled_window_get_vscrollbar(...)) but
this only works after the widget has been shown so my guess is that I 
need to hook into the size allocation
strategy used by GtkTable, is this the best approach?

Thanks
Per