This is actually a bug in LightDM which makes assumptions about the home directory not changing and hardcoding paths to the Xauthority file.
There is also a small fix for libpam-mklocaluser necessary so it runs before the pam_systemd module so that systemd/DBus started processes have the correct home directory. If you want to test, you need to rebuild lightdm with the patch from https://github.com/canonical/lightdm/pull/323 and change the libpam-mklocaluser pam-config priority. The latter is necessary for other display managers as well. -- Guido Berhoerster
