Z-index doesn't work with flex elements? [duplicate]

Question

I'm trying to have two columns, one being a menu which can expand and overlap the other column. But I used a flex element to wrap these columns and my menu expands behind the other element, even with a greater z-index.

The render is something like this:

.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>

See? I want my menu to overlap the rest of my page when expanding. How can I do that?

Breadth

Answer 1

The z-index property only effects elements that have a position value other than static (the default) or are flex items (direct children of display: flex or display: inline-flex).

There are two options to make the z-index work in your case:

1. Set the z-index to the 1st .maincolumn, which is a flexbox item:

   .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;}

<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>

or

2. Set position: relative to nav:

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

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

nav {
  position: relative;
  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>Titre</h2>
      <p>This is a text.</p>
    </div>
  </div>
</div>

Quillet

Simple way to allow z-indexes on a flex container is to wrap the nav in a position relative and then wrap everything below it in a <main> container. <nav><ul><li><a href="" title=""></li><li><a href="" title=""></li></ul></nav> <main> <div =flex-container>wxyz content</div> </main> nav{position: relative;z-index:9999}main{position: relative; z-index:1} div{display: flex} – Batory

Answer 2

Flex items are only direct children of flex-container.

Flex items respect z-index order, but you are applying z-index not to flex-items but to their descendants.

From w3c flexbox spec:

Flex items paint exactly the same as inline blocks CSS21, except that order-modified document order is used in place of raw document order, and z-index values other than auto create a stacking context even if position is static.

So for this to work you should apply greater z-index to your first flex item. Demo:

.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>

Defrock