[CSS] fixed 子元素的 z-index

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