It seems that the main reason why overlay scrollbars didn't work is because I have configured my system to boot to console first. I then use a custom .xinitrc script to start xfce:
#!/bin/sh
case "$1" in
xfce4)
xscreensaver -no-splash &
exec ck-launch-session xfce4-session
;;
awesome)
exec awesome
;;
esac
For some reason, when I used the command xinit xfce4 under Xubuntu 12.04 and previous versions, the overlay scrollbars appeared. However, it seems that 12.10 doesn't load this GTK module, so I had to manually execute this line declare -x GTK_MODULES="overlay-scrollbar". I ended up adding it to my .bashrc file so that I don't have to type it every time.
I got this idea from the /etc/X11/Xsession.d/81overlay-scrollbar file which says:
# This file is sourced by Xsession(5), not executed.
if [ -z "$GTK_MODULES" ] ; then
GTK_MODULES="overlay-scrollbar"
else
GTK_MODULES="$GTK_MODULES:overlay-scrollbar"
fi
so I guess that this file used to be executed on each start of the X windows syst, but now it isn't used because the GTK_MODULES variable wasn't set after start.