What is the current state of the law on the "but for" test for causation?

Having set out the "primary test" for causation, the Chief Justice recognized that it may be appropriate to employ a "material contribution" analysis where the "but for" test is totally unworkable. She wrote, at paras. 24‑28: … [I]n special circumstances, the law has recognized exceptions to the basic "but for" test, and applied a "material contribution" test. Broadly speaking, the cases in which the "material contribution" test is properly applied involve two requirements. First, it must be impossible for the plaintiff to prove that the defendant's negligence caused the plaintiff's injury using the "but for" test. The impossibility must be due to factors that are outside of the plaintiff's control; for example, current limits of scientific knowledge. Second, it must be clear that the defendant breached a duty of care owed to the plaintiff, thereby exposing the plaintiff to an unreasonable risk of injury, and the plaintiff must have suffered that form of injury. In other words, the plaintiff's injury must fall within the ambit of the risk created by the defendant's breach. In those exceptional cases where these two requirements are satisfied, liability may be imposed, even though the "but for" test is not satisfied, because it would offend basic notions of fairness and justice to deny liability by applying a "but for" approach. These two requirements are helpful in defining the situations in which an exception to the "but for" approach ought to be permitted. Without dealing exhaustively with the jurisprudence, a few examples may assist in demonstrating the twin principles just asserted. One situation requiring an exception to the "but for" test is the situation where it is impossible to say which of two tortious sources caused the injury, as where two shots are carelessly fired at the victim, but it is impossible to say which shot injured him: Cook v. Lewis, 1951 CanLII 26 (SCC), [1951] S.C.R. 830. Provided that it is established that each of the defendants carelessly or negligently created an unreasonable risk of that type of injury that the plaintiff in fact suffered (i.e. carelessly or negligently fired a shot that could have caused the injury), a material contribution test may be appropriately applied. A second situation requiring an exception to the "but for" test may be where it is impossible to prove what a particular person in the causal chain would have done had the defendant not committed a negligent act or omission, thus breaking the "but for" chain of causation......