What is the current state of the law on the meaning of the "serious question" in the test?

In Re Attorney-General Of Manitoba v. Metropolitan Stores (Mts) Ltd. et al. (1987), 1987 CanLII 79 (SCC), 38 D.L.R.(4th) 321 (S.C.C.), Beetz J., speaking on behalf of the court, stated at pp. 333-334: In the case at bar, it is neither necessary nor advisable to choose, for all purposes, between the traditional formulation and the American Cyanamid description of the first test: the British case-law illustrates that the formulation of a rigid test for all types of cases, without considering their nature, is not to be favoured. . . . In my view, however, the American Cyanamid “serious question” formulation is sufficient in a constitutional case where, as indicated below in these reasons, the public interest is taken into consideration in the balance of convenience. But I refrain from expressing any view with respect to the sufficiency or adequacy of this formulation in any other type of case.