Clarify how to cancel a navigation #3447
Description
@rwaldron and I reasoned a bit around these tests web-platform-tests/wpt#8558 and found a possible problem with how HTML cancels navigation attempts. It won't remove any "navigate" tasks that still exist on the task queue. Is that intentional?
Consider this test
https://github.com/w3c/web-platform-tests/pull/8558/files#diff-cc01cb4065c5670fb1bf4b19c88fd562
click() will do
https://html.spec.whatwg.org/#the-a-element:following-hyperlinks-2-2
and step 14 in
https://html.spec.whatwg.org/#following-hyperlinks-2
Queue a task to navigate the target browsing context to resource. If replace is true, the navigation must be performed with replacement enabled. The source browsing context must be source.
Note "queue a task".
Then the test also has
onclick="document.querySelector('iframe').src='javascript:`<script>parent.verifyNavigation(false);</script>`';"
https://html.spec.whatwg.org/#the-iframe-element:attr-iframe-src-4
https://html.spec.whatwg.org/#otherwise-steps-for-iframe-or-frame-elements
which step 4 says to navigate, without queueing a task:
Navigate the element's nested browsing context to resource.
So the iframe src navigation starts first (and it will queue another task for https://html.spec.whatwg.org/#javascript-protocol ). Then the hyperlink navigation starts, which cancels the src navigation.
If the above analysis is correct and the behavior is intentional, we should probably clarify in the spec that canceling navigations does not mean to remove tasks from the task queue. OTOH, if removing tasks is a thing browsers do, we should specify that. Maybe each browsing context can have an internal slot to track the most recent navigation to cancel, and have an abstract operation that handles queued task case as well as started case.
Places where the spec cancels navigations:
https://html.spec.whatwg.org/multipage/browsing-the-web.html#navigate
step 6:
Cancel any preexisting but not yet mature attempt to navigate browsingContext, including canceling any instances of the fetch algorithm started by those attempts. If one of those attempts has already created and initialized a new Document object, abort that Document also. (Navigation attempts that have matured already have session history entries, and are therefore handled during the update the session history with the new page algorithm, later.)
https://html.spec.whatwg.org/#traverse-the-history-by-a-delta
step 5.1:
If there is an ongoing attempt to navigate specified browsing context that has not yet matured (i.e. it has not passed the point of making its Document the active document), then cancel that attempt to navigate the browsing context.