scrolled window vadj not being set

[Rob Clack <rnc sanger ac uk>]

My application allows the user to split the display as many times as she 
wants.  Each time she does so, I add an hpane or vpane as child2 to the 
existing hpane or vpane, then add a scrolled window.  (I might not do it 
in precisely that order, but I'm hoping that won't matter.)

For various reasons, I need to know the size of the display in pixels, 
which is where the problem arises.  I use gtk_window_set_default_size() 
to control the initial display (size_request would prevent the user from 
shrinking the window) and gtk_scrolled_window_get_vadjustment() to get 
the vertical height.

On initial display, the height in the adjustment is correct, but when I 
split the display, the exact same piece of code returns an adjustment 
containing mostly zeroes.  Specifically, Upper is set, but step and page 
increments and page size are all zero.

How can I reliably obtain the display height and width in pixels?

My GTK is 2.0 and I'm running Red Hat 9.

Thanks
Rob Clack