It looks like _check_for_correct_username is supposed to ensure that the username starts with a lowercase letter or underscore (but it's not doing so).

I can trigger the message The username you entered is invalid to appear below the input field by starting a username without a lowercase letter or underscore. I am not sure why it is not working for you.

Seems like skip would be the intended outcome when pressing enter at an empty input field.

Did you mean, why is an empty string not triggering the validator? The validator will not be used when text is an empty string.

Ah, yes, that was the confusing part. I assumed the validator function would run on all inputs :).

An empty string is essentially no input, so there is nothing to validate. Makes sense to me anyway.