Z-index不适用于flex元素？

Question

我正在尝试有两列,一个是可以扩展并与另一列重叠的菜单.但是我使用了一个flex元素来包装这些列,我的菜单扩展到了另一个元素后面,即使有更大的元素z-index.

渲染是这样的:

.main {
  font-family: 'Open Sans', Helvetica, sans-serif;
  display: flex;
  background-color: #99baef;
}

nav {
  height: 100%;
  width: 8em;
  background-color: black;
  z-index: 1;
}

.navbox {
  box-sizing: border-box;
  background-color: black;
  color: white;
  height: 4em;
  width: 100%;
  text-align: center;
  line-height: 4em;
  border-top: 1px dotted #99baef;
  transition: all 1s;
}

.navbox:hover {
  width: 130%;
  border-top: none;
  background-color: #4a77c4;
  color: black;
}

.container {
  padding: 1em;
}

a {text-decoration: inherit;}

<div class="main">
  <div class="maincolumn">
    <nav>
      <a href="">
        <div class="navbox">
          Nav 1
        </div>
      </a>
      <a href="">
        <div class="navbox">
          Nav 2
        </div>
      </a>
    </nav>
  </div>
  <div class="maincolumn">
    <div class="container">
      <h2>Title</h2>
      <p>This is a text.</p>
    </div>
  </div>
</div>

看到？扩展时,我希望我的菜单与页面的其余部分重叠.我怎样才能做到这一点？

Answer 1

Flex项目只是flex-container的直接子项.

Flex项目遵循z-index顺序,但您z-index不是应用于flex项目,而是应用于它们的后代.

来自w3c flexbox规范:

Flex的项目漆完全一样,内嵌块CSS21,除了order修饰的文档顺序代替原文档顺序使用,而z-index其它值比auto创建堆叠环境,即使position是static.

因此,为了使其工作,您应该z-index对您的第一个弹性项目应用更多.演示:

.main {
  font-family: 'Open Sans', Helvetica, sans-serif;
  display: flex;
  background-color: #99baef;
}

.maincolumn:first-child {
  z-index: 1;
}

nav {
  height: 100%;
  width: 8em;
  background-color: black;
}

.navbox {
  box-sizing: border-box;
  background-color: black;
  color: white;
  height: 4em;
  width: 100%;
  text-align: center;
  line-height: 4em;
  border-top: 1px dotted #99baef;
  transition: all 1s;
}

.navbox:hover {
  width: 130%;
  border-top: none;
  background-color: #4a77c4;
  color: black;
}

.container {
  padding: 1em;
}

a {text-decoration: inherit;}

<div class="main">
  <div class="maincolumn">
    <nav>
      <a href="">
        <div class="navbox">
          Nav 1
        </div>
      </a>
      <a href="">
        <div class="navbox">
          Nav 2
        </div>
      </a>
    </nav>
  </div>
  <div class="maincolumn">
    <div class="container">
      <h2>Title</h2>
      <p>This is a text.</p>
    </div>
  </div>
</div>

Answer 2

该z-index属性仅影响位置值不是静态值（默认值）或弹性项目（display: flex或的直接子项）的元素display: inline-flex。

有两种选择可以使z-index您的工作顺利进行：

1. 将设置z-index为1st .maincolumn，这是一个flexbox项：

   .maincolumn:first-child {
     z-index: 1;
   }
   

.maincolumn:first-child {
  z-index: 1;
}

.main {
  font-family: 'Open Sans', Helvetica, sans-serif;
  display: flex;
  background-color: #99baef;
}

.maincolumn:first-child {
  z-index: 1;
}

nav {
  height: 100%;
  width: 8em;
  background-color: black;
}

.navbox {
  box-sizing: border-box;
  background-color: black;
  color: white;
  height: 4em;
  width: 100%;
  text-align: center;
  line-height: 4em;
  border-top: 1px dotted #99baef;
  transition: all 1s;
}

.navbox:hover {
  width: 130%;
  border-top: none;
  background-color: #4a77c4;
  color: black;
}

.container {
  padding: 1em;
}

a {text-decoration: inherit;}

要么

2. 设置position: relative为nav：

   nav {
      position: relative;
      height: 100%;
      width: 8em;
      background-color: black;
      z-index: 1;
    }
   

<div class="main">
  <div class="maincolumn">
    <nav>
      <a href="">
        <div class="navbox">
          Nav 1
        </div>
      </a>
      <a href="">
        <div class="navbox">
          Nav 2
        </div>
      </a>
    </nav>
  </div>
  <div class="maincolumn">
    <div class="container">
      <h2>Titre</h2>
      <p>This is a text.</p>
    </div>
  </div>
</div>

nav {
   position: relative;
   height: 100%;
   width: 8em;
   background-color: black;
   z-index: 1;
 }