[kevingadd]

Even if they can't use contain, this is a self-inflicted problem - if you look at the screenshot, other elements + a drop shadow overlap the content list, which means it can't trivially be efficiently scrolled by a compositor, and it's quite reasonable for the browser to assume that it can affect layout. If it were a properly isolated box in the page without overlap with overflow turned on I bet this would have already worked properly.

[Twirrim]

But the fancy graphics! Got to have the fancy graphics! Form > Function.

[pcwalton]

I don't think this is a reasonable assumption for the browser at all. It's 2021; WebRender has proved that a better design is possible.

[kevingadd]

Sure, but are Chrome and Safari going to do a WebRender-style renderer anytime soon? WebRender is great but in practice it affects at most (generously) 20% of the web audience. Optimize for the browser you have, or maybe the browser coming next year, not the one your customers will have in 5 years.