Function
While the surface is waiting, the button stays lit, because its arc and pending label are the wait.
The button follows the surface's wait, not only the request's: sign-in goes on waiting after its credentials are accepted, through the navigation, and the button must not drop back to its resting label while the beam is still sweeping.
The end of every account form's fields: the announced error of the last refused attempt, and the submit button, which stays disabled until the form may be sent. A challenged form places its Turnstile challenge on its own, with
AccountChallenge.