Would you be so kind as to type startxfce4 are you using a nvidia card or the intel card or ATI if nvidia you should install from extra the
for Nvidia. If you read this for current CHANGES_AND_HINTS.TXT
you will see that default is set to kwin desktop for composite.
as per above. .
the default setting /.kde/share/config/kwinrc in the [Compositing] section. I have Backend=Xrender for the nouveau driver
this has caused a lot of problems. in current with certain cards. this needs to be set to.
or do as I did delete it. You need to read this thread if running current
. but the problem you are having was dealt with in November 2010.