When there are multiple sessions, the process of switching to another one is unintuitive/unfriendly and confusing. See the scenario in "Steps to Reproduce" Reproducible: Always Steps to Reproduce: 1. Alice and Bob both have a existing plasma session. Bob is active and locks screen. 2. After some time Alice comes to the computer. She does not know whether her session is still there, so she must first choose "Change session" option and confirm by clicking button. 3. She checks list of all active sessions. She does not understand the cryptic lables of the virtual displays, but at least there are user names. 4. If she finds her session, she activates it. 4a. Because that is what Bob did before, she sees again list of active sessions (so she is not sure what happened). She must click "cancel". Find Her name in the list of options, and after all that she can finally enter her password and get to her session. 5. If she does not find her session, she must click "cancel" button, select "New session" option and confirm by clicking button. 5a. Now she can choose her user name and enter password to begin new session. If Alice skips points 4 an 4a, and just starts new session, it may happen that she unwillingly starts second session and then does not understand why she can't start e.g. firefox or thunderbird. Actual Results: Confused and angry user. Expected Results: I suggest to make lockscreen look exactly as login greeter, with preselected user who's session is "behind" the lock screen. So if Alice comes to computer, she just selects her user name/avatar and enters password. If she has no active session on the computer, new one is created for her, otherwise she is switched to her active session. My proposal allows each user to have only one session on one computer. IMHO user needs multiple sessions only in very special cases. If you think it is important to allow that, I suggest to make it configurable. Default would be one session per user. Power users who needs more, would'n have problem to make that change in any kind of configuration.
CC'ing kbroulik
Since 5.5 the User Switcher on the Lock Screen shows the proper user name and the user avatar if set. Also, it won't offer to switch between users if only one user is logged in. We could potentially automatically cancel out of the "switch user" UI and back to the lock screen after 1 minute or so. Also, we prevent a user from logging in multiple times from the login screen, at least, you can log in on the console as often as you want and that's nothing we can influence or want to change.