How does one switch users in Unity? (i.e., in the GUI, not
I really want to launch another X session using a different WM, without abandoning the current one.
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In a terminal:
sudo apt-get install xfswitch-plugin
Or via Software Center:
I saw that on How can I switch users from within XFCE?
Using Ctrl+Alt+F1 (or others F* works for me; Ubuntu 18.04 && gdm (default) as login manager).
Hypothesis: If I am not wrong, gdm will after login lunch new X11 (or wayland) server instance and put it on some unused terminal (ctrl+alt+F[1,2,3,4,5,6,7]). So you can leave your session (actually whole graphical server) on where it is (probably ctrl+alt+F2) without any touch (for example i3wm by default does not lock its session) and show login screen, which is always on first virtual terminal.