Implementing a time out for fallback UX

[ianbjacobs]

A developer has told me that it would be useful to close the fallback UX after a timeout rather than leave it open indefinitely (e.g., if the UX has created confusion and the user simply leaves the window).

@stephenmcgruer pointed out that WebAuthn encourages [1] developers to leverage the DOM specification's AbortController interface [2] and thought we might be able to do the same thing for this use case.

[1] https://www.w3.org/TR/webauthn-2/#sctn-abortoperation
[2] https://dom.spec.whatwg.org/#abortcontroller

[stephenmcgruer]

Perhaps obvious since I'm already quoted in the OP, but I think this is a good idea (assuming folks need timeout support) :D.

cc @nickburris to put this on your radar