Comment 2 for bug 1471996

Added the -flatten option for shells like Unity8 that can't deal with multiple windows. In flatten rootless mode, Xmir will do its own compositing and window management, exposing only a single Mir surface...

https://git.launchpad.net/~xmir-team/xorg-server/+git/xmir/commit/?id=0355d3e3fd2ff1f26aae7a8673b3fab3c03e8d7e