XFce and Compiz : xfce doesn't manage the desktop
 

Hello,

I decided to use Compiz-fusion with XFce then I read the gentoo howto about that.
(http://gentoo-wiki.com/HOWTO_compiz-fusion)
But now when I run xfce with (and even without) compiz I get a black screen not managed by xfce.
In order to activate the xfce desktop managing I have to run xfce-session-show -> desktop -> and activate the support
It works but never stay activate ! The next time I run xfce I'll get the same black screen .....
(I - do - save sessions for futur logins)

Do you know how can I force xfce to manage the desktop thanks to command lines that I can run ?
Or maybe it's something else ....

I don't know how resolve that problem ...
Can you help me ?

thanks,