Deliberately inescapable — backdrop, Escape and browser Back are
all refused, so the only exits are the two buttons. Login fires many
authenticated requests at once and each will 312 in turn, so a dismissable
sheet just re-summons itself on the next response; and unlike the 310 device
dialog there is no meaningful "cancel", since the account can't be used until
the invitations are answered. onCancel is therefore unreachable from here.
312 resolution: pending parent (coach / facility) invitations. The user accepts a subset; everything left unselected is sent back as an explicit decline. Then the interrupted request resumes. Port of the iOS
InvitationHandleDialog.