There is no way to cancel a user switch, apart from logging in again as the same user, but this is unintuitive.
Choosing Options > Quit has the effect of canceling
A user won't find that! The login box has "OK" and "Start again". It needs a "Cancel switch user" button too.
As you have already discovered, selecting the same user again also gives the effect of canceling. C-A-Backspace works, too.
Okay, but a "normal" user wouldn't find this.
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