1. 现象
下拉框设置了 z-index 为 1000，却没有遮住底下 z-index 为 5 的内容。
1.1 最简示例
（1）HTML
<div class="fixed">
<div class="dropdown">
z-index: 1000;
</div>
</div>
<div class="error">
z-index: 5;
</div>
（2）CSS
body {
margin: 0;
}
.fixed {
position: fixed;
z-index: 3;
}
.dropdown {
position: absolute;
background: gray;
width: 300px;
height: 300px;
z-index: 1000;
}
.error {
position: absolute;
background: red;
width: 100px;
height: 100px;
left: 250px;
top: 250px;
z-index: 5;
}
1.2 效果
其中灰色部分表示下拉框，z-index为1000，
红色部分，z-index为5。
2. 原因
Chrome 22 之后，position为fixed的元素会创建一个新的层叠上下文（stacking context），
而子元素的 z-index 值，只在父级层叠上下文中才中有意义。
Importantly, the z-index values of its child stacking contexts only have meaning in this parent.
Everyone knows and loves the z-index for determining depth ordering of elements on a page. Not all z-indexes are created equal, however: an element's z-index only determines its ordering relative to other elements in the same stacking context.
Within a local stacking context, the z-index values of its children are set relative to that element rather than to the document root. Layers outside of that context — i.e. sibling elements of a local stacking context — can't sit between layers within it.
3. 修复方案
.fixed {
...
z-index: 6;
}
参考
Stacking Changes Coming to position:fixed elements
The Stacking Context
CSS stacking contexts: What they are and how they work
