## Observation

after several switch user, the user cannot login to the desktop event though
the password had accepted. 

## Reproducible

1. two users test1 and test2
2. use test1 to login into desktop
3. select "switch user" to the gdm
4. select test2 to login
5. select "switch user" again to the gdm
6. select test1 to login. it cannot login even if the password accepted. 

## Further details

If send key "ctrl+alt+F2" and quit this session, then back to tty7. it would
good. 

Ubuntu 18.04 Gnome 3.28 had fixed this bug. Please reference: 
bln#1769221: https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1769221,
bln#1766137: https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1766137