Are you involved in xfce4 development? I'm wondering if anybody's working on the "todo" parts concerning xrandr-1.2 in xfce4, and/or what the plans are for direct, non-XINERAMA support for xrandr multi-head in xfce4.
If I understood agd5f correctly, XINERAMA was really meant to be used to get an extended desktop with zaphod mode, but some googling tells me it is also used on top of xrandr to report desktop size and configuration to the WM. But it is meant to be replaced completely by the xrandr API?
Anyway, with only one monitor attached:
$ xdpyinfo -ext XINERAMA
XINERAMA extension not supported by xdpyinfo
number of extensions: 30
$ xdpyinfo -ext XINERAMA | grep head
So that might be the problem. How do I get the x server to report monitor info via XINERAMA?
(BTW, I'm happy using the xrandr tool to configure the outputs, i.e. I don't really need to use the xfce4 settings manager, but xfce4 still need to be aware of the current setup, of course.)