Today morning something strange has happened and suddenly Alt+F1 shortcut takes me to the screen where I have to pick my user - I don't have to put my password again and every windows is still opened. I's not like locking up my computer with Win+L, where I still have to put my password.
It's very strange because I use Alt+F1 every day many many times (IntelliJ shortcut) and it has never happened.
Any ideas how can I disable it? It's nowhere to be found in Shortcuts in Settings.
You have probably pressed Ctrl + Alt + F1.
Ctrl + Alt + F# switches through the TTYs.
F1 switches to the X session with the display manager (gdm) running the login screen.
F2 switches to the first logged in user,
F3 switches to the second logged in user,
... and so on.
This makes you easily switch between logged in users on a multi-user system.
Any TTY where no user is logged in will run a console (text) TTY.
When you go to gdm (F1) and log in, you will automatically be sent back to the TTY you are already logged in (in your case probably TTY-2).
Try it, you can always switch back to your "normal" session using Ctrl + Alt + F2.
In older versions of Ubuntu (e.g. 16.04), X was running on TTY7 per default (F7) and the display manager lightdm was not on a separate TTY.
