It seems that the intuitive way to join the lobby for people is to just click it, which causes the default control scheme to be the mouse. This is not ideal because the mouse controls are not great, and were only added as a 4th option when the keyboard is full with 3 players.
But rather than remove it entirely, I have instead added a warning, because I've also seen it used for accessibility reasons, allowing those that can only use a mouse to play too, for example: https://www.youtube.com/watch?v=8T7-i79xCI4
Changelog:
ADDED - Warning when you try to join the lobby with the mouse that it is not meant to be default
FIXED - “Join” is capitalised in the lobby when it is mid sentence.