true while a host prompt is on screen awaiting the user.
The kind of prompt on screen — the innermost one, when they nest (an add-secret modal
that reaches the passkey unlock reports passkey). null when not awaiting.
Date.now() when the current wait began — the start of the whole continuous span, not
of the innermost prompt. null when not awaiting.
Whether the host is waiting on the user right now, and for what.