In case anyone is thinking it, switching Xmir to glamor to work around this bug is not a good idea. We default to glamor in Xmir on desktop and it's still too buggy (work in progress). The full list of issues for Xmir+glamor is here: https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bugs?field.tag=xmir
In case anyone is thinking it, switching Xmir to glamor to work around this bug is not a good idea. We default to glamor in Xmir on desktop and it's still too buggy (work in progress). The full list of issues for Xmir+glamor is here: https:/ /bugs.launchpad .net/ubuntu/ +source/ xorg-server/ +bugs?field. tag=xmir