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Stacking context example 3 - CSS: Cascading Style Sheets 编辑

Stacking context example 3

This last example shows problems that arise when mixing several positioned elements in a multi-level HTML hierarchy and when z-indexes are assigned using class selectors.

Let's take as an example a three-level hierarchical menu made from several positioned DIVs. Second-level and third-level DIVs appear when hovering or clicking on their parents. Usually this kind of menu is script-generated either client-side or server-side, so style rules are assigned with a class selector instead of the id selector.

If the three menu levels partially overlap, then managing stacking could become a problem.

The first-level menu is only relatively positioned, so no stacking context is created.

The second-level menu is absolutely positioned inside the parent element. In order to put it above all first-level menus, a z-index is used. The problem is that for each second-level menu, a stacking context is created and each third-level menu belongs to the context of its parent.

So a third-level menu will be stacked under the following second-level menus, because all second-level menus share the same z-index value and the default stacking rules apply.

To better understand the situation, here is the stacking context hierarchy:

root stacking context
- LEVEL #1
  - LEVEL #2 (z-index: 1)
    - LEVEL #3
    - ...
    - LEVEL #3
  - LEVEL #2 (z-index: 1)
  - ...
  - LEVEL #2 (z-index: 1)
- LEVEL #1
- ...
- LEVEL #1

This problem can be avoided by removing overlapping between different level menus, or by using individual (and different) z-index values assigned through the id selector instead of class selector, or by flattening the HTML hierarchy.

Note: In the source code you will see that second-level and third level menus are made of several DIVs contained in an absolutely positioned container. This is useful to group and position all of them at once.

Example source code

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head><style type="text/css">

div { font: 12px Arial; }

span.bold { font-weight: bold; }

div.lev1 {
   width: 250px;
   height: 70px;
   position: relative;
   border: 2px outset #669966;
   background-color: #ccffcc;
   padding-left: 5px;
}

#container1 {
   z-index: 1;
   position: absolute;
   top: 30px;
   left: 75px;
}

div.lev2 {
   opacity: 0.9;
   width: 200px;
   height: 60px;
   position: relative;
   border: 2px outset #990000;
   background-color: #ffdddd;
   padding-left: 5px;
}

#container2 {
   z-index: 1;
   position: absolute;
   top: 20px;
   left: 110px;
}

div.lev3 {
   z-index: 10;
   width: 100px;
   position: relative;
   border: 2px outset #000099;
   background-color: #ddddff;
   padding-left: 5px;
}

</style></head>

<body>

<br />

<div class="lev1">
<span class="bold">LEVEL #1</span>

   <div id="container1">

      <div class="lev2">
      <br /><span class="bold">LEVEL #2</span>
      <br />z-index: 1;

         <div id="container2">

            <div class="lev3"><span class="bold">LEVEL #3</span></div>
            <div class="lev3"><span class="bold">LEVEL #3</span></div>
            <div class="lev3"><span class="bold">LEVEL #3</span></div>
            <div class="lev3"><span class="bold">LEVEL #3</span></div>
            <div class="lev3"><span class="bold">LEVEL #3</span></div>
            <div class="lev3"><span class="bold">LEVEL #3</span></div>
            <div class="lev3"><span class="bold">LEVEL #3</span></div>
            <div class="lev3"><span class="bold">LEVEL #3</span></div>
            <div class="lev3"><span class="bold">LEVEL #3</span></div>
            <div class="lev3"><span class="bold">LEVEL #3</span></div>
            <div class="lev3"><span class="bold">LEVEL #3</span></div>

         </div>

      </div>

      <div class="lev2">
      <br /><span class="bold">LEVEL #2</span>
      <br />z-index: 1;
      </div>

   </div>
</div>

<div class="lev1">
<span class="bold">LEVEL #1</span>
</div>

<div class="lev1">
<span class="bold">LEVEL #1</span>
</div>

<div class="lev1">
<span class="bold">LEVEL #1</span>
</div>

</body></html>

Original Document Information

Author(s): Paolo Lombardi
This article is the English translation of an article I wrote in Italian for YappY. I grant the right to share all the content under the Creative Commons: Attribution-Sharealike license.
Last Updated Date: July 9, 2005

Note: the reason the sample image looks wrong - with the second level 2 overlapping the level 3 menus - is because level 2 has opacity, which creates a new stacking context. Basically, this whole sample page is incorrect and misleading.

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