I applied to a machine that was definitely broken, logged out and loggedback in and things work with that added argument.
If you're still doing stuff in, say, an hour from now, go for a sync and update the system at that time. Remove the -rrxinerama workaround and create a new session without it.
I've pushed a fix for the wrong screen size with RRXinerama. Packages are currently building on launchpad. Upload ETA ~30 minutes.
Mihai