I think Ubuntu already runs a separate X or Wayland instance for the login screen. If you press Ctrl+Alt+F2 / F3, you can switch between the login screen and th

I think Ubuntu already runs a separate X or Wayland instance for the login screen. If you press Ctrl+Alt+F2 / F3, you can switch between the login screen and the current desktop. I don't know if this is the default or if I did something to enable it, though. It could be this only happens if you use the "switch user without logging out" function. Coincidentially, Windows does something similar a lot. UAC prompts run on a different Desktop which is completely separate, the log in screen is separate, as well as the secure attention screen (what you get when you press Ctrl-Alt-Del).