> Which thread are you going to do that work on?

The layout thread.

> It's complicated because layout changes can cause new elements to be created. Elements can be really expensive to create.

Huh? No, they can't. In rare cases, layout changes can cause new render objects to be created (line breaks), but that's not elements.

Not sure how Servo works, but what happens if a list viewport gets bigger and you need to display more list items?

The newly visible list items are already laid out, so we just display them.

That's the kind of thing that would cause a rejection: elements are gong to be created, so this can't be converted to compositing stage operations instead.

Dunno which thread—just an idea.