about:blank iframe fires two load events

[jdm]

<iframe onload="console.log('iframe load')"></iframe>

[gpoesia]

I'd like to investigate this.

[jdm]

@gpoesia That would be really useful! I suspect it's caused by the special case in HTMLIFramElement::process_the_iframe_attributes and ScriptThread::handle_frame_load_event, so we'll probably want to suppress the one invoked by the ScriptThread.

[cbrewster]

The first load event comes from when the iframe is bound to the tree and create_nested_browsing_context is called. This will cause about:blank to be synchronously loaded which will fire a load event (this one is a bit hairy to track down). And then process_the_iframe_attributes will be called.
Since it is being called 1. For the first time 2. Without a src attribute, it will queue a task to fire the load event.

The second load event follows the spec as defined here: https://html.spec.whatwg.org/multipage/embedded-content.html#process-the-iframe-attributes

As @jdm said, we will want to suppress the load event that is fired when we are creating the nested browsing context with the initial about:blank load.

[cbrewster]

@gpoesia how is this going? Is there anything I can do to help?

[gpoesia]

@cbrewster Would you take a look at #16091? I didn't find a better way of testing than waiting for some timeout. It bothers me that this makes the test non-deterministic in theory, but I think there can't be an exact way since we want to assert that "onload won't ever be called again", and that seems undecidable.

[jdm]

#16091 was a work in progress to fix this; I believe #16091 (comment) was the last set of requests but the author stopped responding.