It's not the right place to ask, imo.
This is a small community with no particular interest in Xfce/GTK; the probability someone can help you in an other way than doing your work (i.e. researching the question yourself) is pretty low, I think.
In your place I would probably look at Xfce docs, maybe the docs of a more popular Linux distro using Xfce (Xubuntu for example), if you find nothing replicate the problem in a virtual machine running one of those more popular distros and go ask in their support forums.
(amoral shortcut: go to the Ubuntu IRC channel, claim you are running Xubuntu and ask your question right away)