css z-index lost after webkit transform translate3d

Question

I have two absolutely positioned div elements that overlap. Both have set z-index values via css. I use the translate3d webkit transform to animate these elements off the screen, and then back onto the screen. After the transform, the elements no longer respect their set z-index values.

Can anyone explain what happens to the z-index / stack-order of the div elements once I do a webkit transform on them? And explain what I can do to keep the stack-order of the div elements?

Here is some more information on how I am doing the transform.

Before the transform, each element gets these two webkit transition values set via css (I am using jQuery to do the .css() function calls:

element.css({ '-webkit-transition-duration': duration + 's' }); element.css({ '-webkit-transition-property': '-webkit-transform' }); 

The element is then animated using the translate3d -webkit-transform:

element.css({ '-webkit-transform': 'translate3d(' + hwDelta + 'px, 0, -1px)' }); 

Btw, I have tried setting the 3rd parameter of translate3d to several different values to try to replicate the stack-order in the 3d space, but to no luck.

Also, iPhone/iPad and Android browsers are my target browser that this code needs to run on. Both support webkit transitions.

Answer 1

This might be related to: https://bugs.webkit.org/show_bug.cgi?id=61824

Basically when you apply a 3D transform on the z-axis, the z-index can't be accounted for anymore (you're now in a 3 dimensional rendering plane, use different z-values). If you want to switch back to 2D rendering for child elements, use transform-style: flat;.

Answer 2

This is most definitely related to the bug noted by samy-delux. This should only affect any elements which are positioned as absolute or relative. In order to remedy the issue, you can apply the following css statement to every element which is positioned this way and is causing issues:

-webkit-transform: translate3d(0,0,0); 

This will apply the transform to the element without actually doing a transformation, but affecting its render order so it is above the element causing the issue.