Weird rendering bug in desktop webkit (safari/chrome) with absolutely positioned elements

Question

If you look at the video here: http://f.cl.ly/items/2g1a2B3G312D0x0G353W/Render%20Bug%202.mov - you will see the problem in action. Basically, I have something along the following:

<section id="sidenav">
  <h1>TEXT HERE</h1>
  <ul>
    <li>Tab One</li>
    <li>Tab Two</li>
    <li>Tab Three</li>
    <li>Tab Four</li>
  </ul>
  <div id="tab"></div>
</section>

Sidenav is absolutely positioned, like this:

#sidenav {
  position: absolute;
  top: 200px;
  left: 0px;
  width: 770px;
  padding: 30px 0px 20px;
  background: rgba(255, 255, 255, 0.85);
  -webkit-transition: left 0.75s ease-in-out;
  -webkit-backface-visibility: hidden;
  z-index: 10; /* This fixed it. */
}

#sidenav.hidden {
  left: -768px;
}

I have the following jQuery:

$("#tab").click(function(){
  $("#sidenav").toggleClass("hidden");
});

However, the elements inside of the section aren't keeping up with the animation. Whenever I click, they either lag behind or don't move at all. However, they are just ghosts, I can't click them. When I bring the side nav back out, they usually catch up, but sometimes they are broken until I hover over the <li>'s.

Keep in mind, this only happens in Safari/Chrome on the desktop. Safari on the iPad and Firefox on the desktop are working fine.

Thanks! Andrew

---

EDIT WITH FIX:

So apparently adding z-index: 10 (or any z-index) to the sidenav element fixed the problem. Some people were asking for the entirety of my css, so I edited the post to contain it. I'm not sure exactly why z-index fixed this, and I'd love to know why. I'm still offering my bounty to whomever can explain that. Thanks!

Answer 1

So apparently adding z-index: 10 (or any z-index) to the sidenav element fixed the problem. Some people were asking for the entirety of my css, so I edited the post to contain it. I'm not sure exactly why z-index fixed this, and I'd love to know why. I'm still offering my bounty to whomever can explain that. Thanks!

Answer 2

I would prefer to post this as a comment, but since I'm a newbie here, my only option is posting this as an answer. In the video example you posted the hover over the list elements allowed for the display of the arrows, but they did not go away on mouse out. If you are trying to do this purely with css and not have the latent images, you should use hover.

That is detailed in this post: Using only CSS, show div on hover over <a>

That way, if you hide the arrows as the mouse leaves the list element, there will not be any arrow to stay behind when the list slides off the page to the left.