webkit-transform breaks z-index on Safari - html

Question

Problem
I'm trying to make a layer appear like it's a wall falling down, revealing the layer behind it. I've setup two fixed div positions. The "Wall" div has a z-index of 9999, the "Background" div has a z-index of 0;
In Webkit browsers (Safari/IOS) that I've tested, it seems like once the animation starts on the "wall", the z-indexes are lost or ignored, causing the "wall" layer to abruptly disappear behind the background div.
Any ideas on how to preserve the z-indexes of the layers? Thanks in advance!
Example Code
(note: jsFiddle at the bottom)
HTML Code
<div id="wall">
This is the wall
</div>
<div id="background">
This is the background
</div>
<button id="start" style="float: right;">
Flip Down
</button>
Some javascript to enable the button
$('#start').click(function(){
alert('Should Fall Down like a wall, revealing the background');
$('#wall').addClass('animated flipDown');
});
CSS Code (cribbed from animate.css)
#wall{
background-color: #F00;
width: 100px;
height: 100px;
position:fixed;
top:0;
left:0;
z-index: 9999;
}
#background{
background-color: #00F;
width: 100px;
height: 100px;
position:fixed;
top:0;
left:0;
z-index: 0;
}
.animated {
-webkit-animation-duration: 1s;
animation-duration: 1s;
-webkit-animation-fill-mode: both;
animation-fill-mode: both;
}
/*** flipDown ***/
#-webkit-keyframes flipDown {
0% {
-webkit-transform: perspective(400px) rotateX(0deg);
transform: perspective(400px) rotateX(0deg);
-webkit-transform-style: flat;
opacity: 1;
}
100% {
-webkit-transform: perspective(400px) rotateX(90deg);
transform: perspective(400px) rotateX(90deg);
-webkit-transform-style: flat;
opacity: 1;
}
}
#keyframes flipDown {
0% {
-webkit-transform: perspective(400px) rotateX(0deg);
-ms-transform: perspective(400px) rotateX(0deg);
transform: perspective(400px) rotateX(0deg);
opacity: 1;
}
100% {
-webkit-transform: perspective(400px) rotateX(90deg);
-ms-transform: perspective(400px) rotateX(90deg);
transform: perspective(400px) rotateX(90deg);
opacity: 0;
}
}
.flipDown {
-webkit-animation-name: flipDown;
animation-name: flipDown;
-webkit-backface-visibility: visible !important;
-ms-backface-visibility: visible !important;
backface-visibility: visible !important;
-webkit-transform-origin: bottom;
-ms-transform-origin: bottom;
transform-origin: bottom;
}
jsFiddle
http://jsfiddle.net/3mHe2/2/
Check out the differences in Safari vs Chrome.

Answer 1

My rotating element wasn't suitable to have a neighbour to the background, but I fixed it by applying
transform: translateZ(1000px);
transform-style: preserve-3d;
to the parent of the rotating element. Safari now thinks it's 1000px infront of the background.

Answer 2

Found a solution. Hopefully this helps someone in the future.
It turns out that there is a "bug" in the safari versions of webkit. When a 3d css animation is playing, the original z-indexes are lost. Instead, it seems like the animating pieces are put into a separate z-index "group" that is separate from the rest of the z-indexes of the DOM.
The solution is to join the backdrop div and the wall div into the same z-index group by wrapping it in a div with a webkit-transform that doesn't change anything. That causes the backdrop and wall to be children of the wrapper div and the z-indexing of the respective children are preserved.
<div id="wrapper" style="-webkit-transform: translate3d(0px, 0px, 0px);">
<div id="wall">
This is the wall
</div>
<div id="background">
This is the background
</div>
</div>
I believe it is the same or similar issue to this:
css z-index lost after webkit transform translate3d

Answer 3

I ran into this issue and nothing would fix it until i added perspective to the parent container of the item that should be behind.
.wrap{
perspective: 1000px;
}

Answer 4

In my case I was able to solve the issue by applying translateZ to the parent and translate scale to the child.
.parent {
transform: translateZ(22px);
}
.child {
transform: scale(0.955);
}