Load events across portals

[lucasgadani]

We need a way to synchronize events between the embedder and the portal. This could be done with a load event on the portal element that notifies the embedder that the portal has completed loading.

[lucasgadani]

We should also think about whether the portal element should delay the load event of its containing document.

The spec mentions that <iframe>s should delay the load event until they are ready for post-load tasks (https://html.spec.whatwg.org/#the-iframe-element:delay-the-load-event).

[domenic]

Have any recent course corrections changed our decision here, or do we still think a load event is valuable? I'd be inclined to imitate iframes, in general.

[jeremyroman]

The Chromium implementation does have a load event (analogous to iframes) which we found to be very useful for writing tests. I think we probably should have this, and therefore should spec it.

[domenic]

Is it OK to communicate cross-origin load?

[jeremyroman]

We do for iframes. And I think #191 makes a compelling point that it's difficult to hold properly without some basic signals about the readiness of the content.

[jakearchibald]

We do for iframes.

Although iframes wouldn't be first-party, and portals might be. Providing the load event of a first-party page would be a new capability that we should probably avoid.

[jeremyroman]

Fair point. Popups probably still do as well, but maybe given the direction frames are going we should either not fire this, or restrict it to same-origin.

We'd previously found it rather inconvenient to know when you could start talking to a portal contents via postMessage (plumbing your own ready message works but gets old quickly, and it's also kinda nice to be able to delay the outer load). But those concerns apply more acutely to portals with which you are permitted to coordinate, so maybe this should be re-evaluated.

[domenic]

Adding "design work needed" since it sounds like we're not sure whether this should be fired for cross-origin portals...

I guess I can still work on a spec PR though that fires it at all times, and we could restrict it later.

[domenic]

I put up a draft for firing the load event.

How do we feel about delaying the load event of the outer document? I'm not sure what the load event is used for these days, so it's hard to say whether it's worth the complexity... I guess I use it when writing tests sometimes.

[jeremyroman]

I don't feel strongly, but if I were an author I think I would expect it to behave similarly to images, frames, etc.