GtkFrame looks different each time the program is started

[Andreas Falkenhahn <andreas falkenhahn com>]

This is really confusing the heck out of me. Please take a look at this
screenshot:

http://www.falkenhahn.com/tmp/shot.png

Sometimes, the GtkFrames in my program look like on the left side, and
sometimes they look like on the right side - or even a mix of the two!
The program is exactly the same. Just running the same program multiple
times yields very different looks of the GtkFrames in the program! How
can that be?

It seems that there are two different designs of GtkFrame:

1) The first one has its label centered at the top of the frame and smoothly
dissolves towards the bottom so that the frame doesn't completely enclose
the GtkFrame's contents.

2) The second design has its label left-aligned at the top of the frame and
draws a border around the complete GtkFrame.

The problem is now that GTK+ seems to choose one of the two designs entirely
at random. I don't see any pattern in which design I'm going to get. It appears
to happen completely at random which is really confusing me.

Can somebody shed a light onto this mystery? What is going on here? Is there
a way to force GTK+ to use a certain design?

I'm using GTK+ 2.24.10 with the Adwaita theme on Linux Mint. On Ubuntu I
don't see this behaviour. On Ubuntu all GtkFrames look exactly the same
and they never change. But on Linux Mint with Adwaita the apparently random
change of GtkFrame designs is confusing the heck out of me...

Somebody please shed some light onto this! Thanks a lot!

-- 
Best regards,
 Andreas Falkenhahn                          mailto:andreas falkenhahn com