I want to make a div (with fixed position) become hidden outside a specified area. This do exactly what I what:

```
<div id="div1" style="position:relative; overflow:hidden;">
     <div id="div2" style="position:relative; z-index:10;">
          <div id="div3" style="position:fixed"> </div>
     </div>
</div>

```

[Here is an example.](http://jsfiddle.net/wS5Rx/2/) (Firefox only)

But I dont understand why `#div2` needs the `z-index` property?

Also, iirc, my example used to work on Chrome, but not now. Why?