Open a combobox with a lot of items that is positioned near the edge of a
screen. The popup is displaced correctly to provide more 'screen estate', but
the room that is expanded is empty. The entries in the popup should move, or the
 popup should be smaller. Testing this at the bottom is probably easiest,
although I've seen it happen in a case where a combobox that was positioned
close to the top of the screen had its last element selected 
Look at the attached screenshot to see what I mean.
This screenshot was taken with the file chooser of Gimp 2.2 for Windows. However
I think it's not a Gtk-Windows bug, I've seen it happen in one of my own
programs on Linux as well (although that was with Gtk 2.2).
A simple test case would probably be a window with only a combobox on it and
with a big number (like 50+) entries.