There needs to be a rule against having to read entire articles or YT videos just so that a reader can understand the point that a commenter is trying to make, but which they cannot explain in their own words, no? It’s very anti-discussion, and how do we even know to what extent commenters are quoting or agreeing with their sources?

Your unit tests should be contravariant to the code its testing so that way your code can be refactored and your existing unit tests can be used AS-IS to test the refactored code.

The Fragile Test Problem arises when your unit tests are covariant to the code its testing, such that a significant refactor breaks the tests - not because the code is necessarily wrong, but the structure of the code is no longer what the unit test was expecting.

While the link in question isn't a good article, for complex subjects like this a long article is much better. The article linked should take several days to create if you are a great writer (again, I would create in a different way, but the content deserves days or even weeks of work to create), and thus cannot fit in a comment box except in a reduced form that doesn't do the idea justice.