how to determine maximum possible window size?

[Chris MacGregor <chris-gtk-list bouncingdog com>]

Hello.  I've been doing some work on a GTK+ app (avidemux - see 
http://www.avidemux.org/) and it's been fun learning GTK+.

We have a situation where we need to determine the largest possible size 
that a window can have and still fit on the screen, after deducting 
space taken up by panels and such, so that we can scale down the preview 
video size but only when necessary and no smaller than needed.

However, I can't find any way to query GTK+ (or GNOME) for the remaining 
available screen real estate.  I can get the screen size, but see no way 
to determine how much is used by panels/etc.

What I have right now is a horrible hack that creates a dummy window, 
maximizes it, makes it visible, waits for the configure event (the real 
one, not the 200x200 one that comes first for some reason), records the 
size reported by the configure event, and destroys the window.  That 
works (mostly), but the has the unpleasant side-effect of "flashing" a 
giant blank window onto the screen, which on less-than-super-fast 
machines is kind of distracting.  And the "mostly" refers to the fact 
that I still need to subtract about 10x10 pixels to allow for window 
borders and other fudgement.

So, is there a better way?  Please, tell me there is a better way (and 
what it is).

Thanks,
   Chris