Iframe with 3D transform, content not clipped

[paulrouget]

Even though both #11151 and #11358 are now fixed, we still have issues with iframe content not being properly clipped.

Both previous test case render correctly, so it's something new.

Working on a reduced test case now.

[paulrouget]

It's 3D transform specific:

<style>
  div, iframe {
    width: 100vw;
    height: 100vh;
    top: 0px;
    left: 0px;
  }
  div {
    perspective: 1000px;
    background: grey;
  }
  iframe {
    display: block;
    transform-origin: left center 0px;
    transform: translate3d(0px, 0px, -200px);
  }
</style>

<div>
  <iframe src="https://duckduckgo.com/?q=foobar&amp;ia=about">
  </iframe>
</div>

@pcwalton any ideas? If that's too hard to fix, I think we can simply move to 2D transforms in browserhtml.

[pcwalton]

I think this is probably something related to DDG's specific use of stacking contexts, similar to #11648.

[cbrewster]

This appears to be fixed with WR2 #12711