z-index chrome bug

Question

I'm experiencing a really annoying bug that seems to only happen on Windows and OS X: the z-index of an element whose parent has fixed position doesn't work on Chrome! I converted my odd situation to a simple code:

html:

<div id="mask">
    &nbsp;
</div>

<div id="box">
    <div class="below-mask circle">
        should be below the mask
    </div>

    <div class="above-mask circle">
        should be above the mask
    </div>
</div>​

css:

body {
    font-family: Verdana;
    font-size: 9px;
    margin: 0px;
}

#box {
    position: fixed;
}

#mask {
    position: absolute;
    left: 0px;
    top: 0px;
    background-color: rgba(0, 0, 0, 0.5);
    width: 100%;
    height: 100%;
    z-index: 9998;
}

.circle {
    position: relative;
    background-color: rgba(255, 204, 0, 0.75);
    border-radius: 75px;
    line-height: 150px;
    margin: 50px;
    text-align: center;
    width: 150px;
    height: 150px;
}

.above-mask {
    z-index: 9999;
}

.below-mask {
    z-index: 9997;
}​

sandbox: http://jsfiddle.net/gibatronic/umbgp/

I tested on Internet Explorer 9, Firefox 15, Opera 12.02 and Safari 5.1.7 on OS X and Windows and all of them displayed as expected. I also tested on Ubuntu 12.10 and it worked just fine for every browser including Chrome! I even tested on Kindle 4 browser and it worked!

I wonder if anyone knows any kind of fix to workaround this issue!

Answer 1

one800higgins's answer is along the right lines. The real answer is that on mobile WebKit and Chrome 22+, position: fixed always creates a new stacking context, even when z-index is auto. So the stacking context hierarchy looks like this:

• document root (z-index 0)
  • #mask (z-index 9998)
  • #box (z-index 0)
    • .above-mask (z-index 9999)
    • .below-mask (z-index 9997)

That means that 9998 is never compared with 9999 or 9997 to determine stacking order. Instead, 9999 is compared with 9997 to determine which of .above-mask and .below-mask is further in front, and then once everything inside #box is stacked in that context, it's treated as a single layer at z-index 0 which gets stacked behind #mask at z-index 9998.

This also explains why @TheNextBillGates's answer of moving #mask inside #box works - because then #mask is in the same stacking context as .above-mask and .below-mask. I highly recommend the above link for more comprehensive details, and you should also see the announcement for the stacking change for fixed elements in Chrome.

Answer 2

I just came across this bug, and its still happening in Google Chrome v26. I could set the z-index as high as I wanted to from code or Chrome's CSS editor and it made no difference (and the element's position was set to absolute). The same z-index setting was working as expected in Firefox and even IE8-IE10. When I switched the parent element from position:fixed to position:absolute then the child element's z-index worked fine in Chrome.