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层叠上下文示例 3
最后一个例子展示了，在多层级的HTML结构中混合了多个定位元素且使用类选择器设置z-index属性时出现的问题。
我们来看一个用多个定位的div实现的三级菜单的例子，二级菜单和三级菜单在鼠标悬停或点击其父元素时才出现，通常这样的菜单在客户端和服务端都是由脚本生成的，所以样式规则不是通过ID选择器设置而是通过类选择器设置。
如果这个三级菜单有部分区域重叠，管理层叠顺序就会成为一个问题。
一级菜单仅仅是相对定位，所以没有创建层叠上下文。
二级菜单相对其父元素（一级菜单）绝对定位，要使二级菜单在所有一级菜单的上方，则需要使用z-index。此时每个二级菜单都创建了一个层叠上下文，而三级菜单也处于其父元素（二级菜单）创建的上下文中。
这样一来，在HTML结构中处于三级菜单后面的二级菜单，则会显示在三级菜单的上方，因为所有的二级菜单都使用了同样的z-index值，所以处于同一个层叠上下文中。
为了能更好地理解这种情况，这里列出了层叠上下文的层次结构:
- 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 #2 (z-index: 1)
- LEVEL #1
- ...
- LEVEL #1
- LEVEL #1
可以通过移除不同级别的菜单之间的重叠，或者使用ID选择器指定独立的（不同的）z-index值，或者减少HTML的层级来解决这个问题。
示例源码
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"https://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>
相关文章
- Stacking without z-index : Default stacking rules
- Stacking and float : How floating elements are handled
- Adding z-index : Using z-index to change default stacking
- The stacking context : Notes on the stacking context
- Stacking context example 1 : 2-level HTML hierarchy, z-index on the last level
- Stacking context example 2 : 2-level HTML hierarchy, z-index on all levels
原文信息
- 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 Creative Commons: Attribution-Sharealike license
- Last Updated Date: July 9th, 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.