When using the command block GUI, the "Done" Button is available in an inscosistent manner.
Concretely, a command block's Done buttons is supposed to be available if there's been an edit in the command field. However, the button's available even'f you haven't made an edit yet, and disappears after writing many spaces, then reappears after entering a character (which does not make it available) and removing it.
This, to me, seems to be very unintended.