Disallow portal creation and activation in the unload handlers

[lucasgadani]

For similar reasons we disallow navigations while unloading, we shouldn't allow portals creation/activation while unloading the document.

[jeremyroman]

Do you happen to have the spec reference for how this is handled for frames handy?

[lucasgadani]

I believe frames don't handle this directly. When a user follows a hyperlink, it uses this algorithm:
https://html.spec.whatwg.org/#following-hyperlinks-2

Since we queue a task on the target's browsing context event loop, which is being discarded, the task never gets executed, which means the navigation never occurs. Maybe @domenic can clarify if this is correct?

[domenic]

Hmm, I'm not sure that's right. First, note that it's not that the event loop gets discarded; it's that the task is associated with a document, and once that document becomes not-fully-active, any tasks involving it are skipped. (Until the document becomes fully active again, which can happen!)

But during-unloading navigation is prevented by https://html.spec.whatwg.org/#navigate steps 4 and 5. Which is not a model of clear and precise spec text, to be clear; ideally there would be an explicit flag set and unset which could get checked.

[lucasgadani]

Ah, that makes sense. Thanks for the pointer!

I think we can probably use a condition similar to steps 4/5 in the navigation algorithm to prevent portals from being created and activated.

[domenic]

I've attempted to clean up HTML here in whatwg/html#5666 so that we can do the same more easily in portals.