iframe content not painted at the right coordinates if the iframe forms a stacking context

[paulrouget]

iframe + absolute:position + z-index: iframe content is painted as if iframe's coordinates were top:0;left:0.

    <style>
    body {
      background: green;
    }
    iframe {
      top: 100px;
      left: 100px;
      border: 50px solid red;
      width: 400px;
      height: 400px;
      position:absolute;
      z-index:1;
    }
    </style>
    <iframe src="http://example.com"></iframe>

[pcwalton]

I wonder if this is what's breaking eBay as well!

[pcwalton]

Also reproduces with transform: translateX(0px) instead of z-index, indicating that it's related to forming stacking contexts.