Bug in CSS3 rotateY transition on Safari?

Question

I am showing a modal popup using CSS3 transitions (largely borrowed from Effeckt.css). It works well in all modern browsers except Safari. In Safari, the movement is OK, but the background-color snaps in unevenly.

This is the code, the problem is visible in Safari on OSX: http://jsfiddle.net/eJsZx/4/

A screenshot of the problem before it resolves itself. You can see that half the div is correctly colored white, half is still transparent.

This is the relevant part of the CSS (.effeckt-show and .md-effect-8 are applied when the button is clicked, to show the modal):

.effeckt-modal {
  visibility: hidden;
  -webkit-backface-visibility: hidden;
  -ms-backface-visibility: hidden;
  -o-backface-visibility: hidden;
  backface-visibility: hidden;
  background: white;
}
.md-effect-8 {
  -webkit-perspective: 1300px;
  -ms-perspective: 1300px;
  -o-perspective: 1300px;
  perspective: 1300px;
  -webkit-transform-style: preserve-3d;
  -ms-transform-style: preserve-3d;
  -o-transform-style: preserve-3d;
  transform-style: preserve-3d;
}
.md-effect-8 .effeckt-modal {
  -webkit-transform: rotateY(-70deg);
  -ms-transform: rotateY(-70deg);
  -o-transform: rotateY(-70deg);
  transform: rotateY(-70deg);
  -webkit-transition: all 500ms;
  -o-transition: all 500ms;
  transition: all 500ms;
  opacity: 0;
}
.effeckt-show.md-effect-8 .effeckt-modal {
    -webkit-transform: rotateY(0deg);
    -ms-transform: rotateY(0deg);
    -o-transform: rotateY(0deg);
    transform: rotateY(0deg);
    opacity: 1;
}

Answer 1

As far as I can tell it's a bug, yes, Safari is rendering intersection where it shouldn't.

For some time I thought Safari is doing it right by always rendering intersection of elements, but as far as I understand the specs, only elements in the same 3d rendering context should intersect, and that would be children of elements with a transform-style of preserve-3d.

So far the only workaround I found (only tested on Windows yet where Safari shows the same behaviour) is to translate the underlying elements away on the z-axis. Without perspective being applied it won't actually translate, but Safari/Webkit seems to think it does (which probably is because it mistakenly treats the element as if it were in the same 3d rendering context as the actually transformed dialog) and so the elements do no longer intersect.

.effeckt-overlay {
    position: fixed;
    width: 100%;
    height: 100%;
    visibility: hidden;
    top: 0;
    left: 0;
    opacity: 0;
    -webkit-transition: 500ms;
    -o-transition: 500ms;
    transition: 500ms;
    z-index: 1000;
    background: rgba(0, 0, 0, 0.5);

    -webkit-transform: translateZ(-1000px);
}

http://jsfiddle.net/eJsZx/5/