Comment 8 for bug 1542028

Done, in a series of commits:
https://git.launchpad.net/~xmir-team/xorg-server/+git/xmir/

Unfortunately how well it works depends on the window manager. It worked immediately with no window manager. And took some more effort to work with Matchbox window manager. Also seems to mostly work with Compiz, but not at all with Metacity for some reason.

If you find it's not working as desired, please take a snapshot of:
   $ xwininfo -root -tree
and send it to me.