Comment 6 for bug 1605012

May also be helpful:

When I lock and try to change language in the login screen (Alt+Shift), it flashes as described showing me the desktop for a moment.
When I then log in successfully, Shut Down option doesn't work as expected, throwing me out to login screen.

But if I lock and NOT try the language switch, then Shut Down works fine.

Anyways, Ubuntu and lightDM are awesome :p