Some analysis of whether this "finalizes with an aborted navigation error" often enough. In particular we need to think about the cases listed in whatwg/html#6927 when the SDT or SDN is extended by respondWith().

Example:

appHistory.addEventListener("navigate", e => {
  e.respondWith(new Promise(r => setTimeout(r, 1_000)));
});

await appHistory.navigate("#foo");

// What does this do?
const p1 = appHistory.navigate("#bar");
history.back();

This is a SDN-SDT case, which in the pre-app history world is straightforward: the SDN completes synchronously, and so the SDT then proceeds. (We would end up on #foo.)

In the app history world however, at the time history.back() is called, the navigation to #bar is still "ongoing". That is, location.href is updated to #bar already, but the loading spinner is still spinning, and navigatesuccess hasn't fired, and p1 hasn't settled.

So we have to decide what it means to traverse back while a navigation is still ongoing. IMO the simplest is that the navigation counts as aborted: it didn't reach the end state, as signaled by the developer's promise passed to respondWith().

Indeed, in this case we match the behavior for cross-document navs/traversals: per whatwg/html#6927, XDN + XDT results in the XDN being canceled. So in this case developers could have the intuition "using respondWith() to extend a same-document nav/traversal makes their overlap behave like the corresponding cross-document case".

The app history spec draft, after this PR, gives the desired result. Because of the clause saying "whenever you would normally cancel non-matured cross-document navigations, also inform app history about canceling navigation", we properly reject p1 and fire navigateerror.

Let's consider the reverse case:

appHistory.addEventListener("navigate", e => {
  e.respondWith(new Promise(r => setTimeout(r, 1_000)));
});

await appHistory.navigate("#foo");

// What does this do?
const p1 = appHistory.back();
const p2 = appHistory.navigate("#bar");

This is a SDT-SDN case, which does not have interoperable behavior: Firefox queues up the SDT to happen after the SDN, whereas Chrome ignores the SDN. It's not clear what direction the spec will go in yet, but I would say that if the SDN is ignored, then p2 should reject (and no navigate event should fire for it).

If you look at the corresponding XDT-XDN case, trying to port over our intuition from above, you find from whatwg/html#6927 that the XDN is ignored in both Chrome and Firefox. So that indicates maybe Chrome's behavior is a bit more uniform.

The app history spec draft, after this PR, does not reject p2 in such a situation. App history is only informed when the browser cancels navigations, not when it ignores them. Hmm.