I think it is either backdrop-filter, or sticky positioning (likely the latter), since both of them are enabled by the Experimental web platform features flag and both of them are used on the same element -
<section class="sticky divider-top bg-light" ng-show="results.length && !loading">

#search .sticky {
    position: relative;
    position: -webkit-sticky;
    position: -moz-sticky;
    position: -ms-sticky;
    position: -o-sticky;
    position: sticky; // A suspect.
    top: 0px !important;
    z-index: 9;
    opacity: 0.92;
    backdrop-filter: blur(22.08px); // A suspect.
    z-index: 2 !important;
}

Confirmed with Android (Beta with experimental web platform flag on) and
Windows8 (Canary with experimental web platform flag on).

On Linux (ToT with developer build) the page is rendered almost in black, but it
seems some other compositor/OpenGL issue was happening.

Given above testing, setting OS to ALL.

Thanks phistuck@ for looking in detail.

Over to compositing team for position:sticky.

This looks a lot like a backdrop blur to me. I've also verified that this is caused by the blur by connecting with dev tools and finding the offending style. Removing backdrop-filter: blur(8px) from #search .shadow-box fixed the problem.

I am unable to repro this on Linux (ToT) and Mac Retina (canary). Let me know if you are still seeing it.

I am unable to reproduce, either.

I'm not sure the problem was fixed by the page (developer.apple.com) or
Chrome's experimental web platform features, but as it is not able to reproduce,
let me close this.

Theoretically I can bisect to see at which point the problem is magically
fixed, but as this is "experimental" feature, I don't want to spend much time
on it - unless it is really important/necessary.  Feel free to reassign to me
if you think it is important, or feel free to take this :)