Debug servo build is intermittently unresponsive

[jdm]

When I load https://threejs.org/examples in a debug servo build, often it works as expected. Once in a while it takes a very long time before the hover styles start appearing under the mouse. There is no reported hang from the background hang monitor.

[jdm]

Judging by RUST_LOG=script::scipt_thread, it spends a really long time processing a particular network response event.

[jdm]

It appears to be hanging while reflowing after the DOMContentLoaded event, with a layout that is taking a ridiculously long time.

[jdm]

Servo appears to be issuing a reflow after every single DOM node that is parsed. This is terrible!

[jdm]

Ok, there are a bunch of problems conspiring to make this really bad:

• we unnecessarily perform a synchronous reflow in Document::maybe_dispatch_dom_content_loaded
• the page is using the innerHTML setter, which creates a document fragment and a new document which has a pointer to the original element's window
• the page uses innerHTML on every single link in the list of examples

Taken together this is what happens - for each link in the list of examples, we create a new document, parse a bit of text into it, finish parsing and fire a DOMContentLoadedEvent at the new document, then synchronously reflow the main page without having made any changes to it. Also the synchronous reflow can't be short-circuited because we insist on dirtying every node in the page.

[asajeffrey]

Possibly related? #22926

This sometimes shows itself in the log as a network hang, but it's actually waiting for window events.

[jdm]

This issue reflects a particular edge case involving lots of uses of innerHTML during initial document load. I wouldn't assume too broad an impact.