First, congratulations for this amazing piece of sw.
I am using Xinerama because I need to use different graphic cards for my two monitors and XRandr does not work for this.
When enabled, I get the following artifact (see screnshot). If I disable it, it works. It also works on my laptop with XRandr and two screens perfectly.
I know Xinerama will die, but until XRandr is compatible with multiple cards, many people are stuck with it.
Thanks for the report! Could you try out if it works when disabling the XShm extension? This could be done by inserting the following lines in /etc/X11/xorg.conf:
Option "MIT-SHM" "Disable"