While developing the navigation API, @domenic noticed that cancelation of navigations and history traversals is not very well-defined in the spec.
On the spec side, this will probably be fixed as part of, or after, the session history rewrite.
In the meantime, this directory contains tests which try to match most browsers, or picks one of the potential behaviors.
https://github.com/whatwg/html/issues/6927 discusses these results.