css – Z-index不适用于flex元素？

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

从w3c flexbox spec开始：

因此,为了使其工作,您应该将更大的z-index应用于您的第一个Flex项目.演示：

.main {
  font-family: 'Open Sans',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>