What is the test for enforceability of a forum selection clause?

Forum selection clauses serve a valuable and important purpose, by creating certainty and security in contractual relations. They are “commonly used and regularly enforced”: Douez v. Facebook, Inc., 2017 SCC 33 at para. 24. However, because forum selection clauses have the potential to encroach upon the public sphere by diverting cases from the courts in a particular jurisdiction where they would otherwise be allowed to proceed, the courts retain the discretion to decline to enforce them. In other words, forum selection clauses cannot bind the court or oust its jurisdiction: Douez at para. 24-27.