Why Doesn't z-index Affect Fixed-Positioned Elements?

Element with Fixed Positioning Not Affected by z-index

In CSS, the z-index property controls the stacking order of elements on the page, where higher values move elements to the forefront. However, sometimes fixed positioned elements seem immune to z-index changes.

This behavior is caused by the default static positioning of fixed elements. When an element with fixed positioning is placed within another element with default positioning, it creates a stacking context within that element. Within this context, the fixed element is layered above any other elements, regardless of their z-index.

To resolve this issue, you can add relative positioning to the parent element. This establishes a new stacking context within the parent, allowing the z-index of the fixed element to take effect.

Example:

Consider the following HTML and CSS:

<div>
<pre class="brush:php;toolbar:false">#over {
  width: 600px;
  z-index: 10;
}

#under {
  position: fixed;
  top: 5px;
  width: 420px;
  left: 20px;
  border: 1px solid;
  height: 10%;
  background: #fff;
  z-index: 1;
}

In this case, the #under element will remain on top of the #over element, even though the latter has a higher z-index. To fix this, add relative positioning to the #over element:

#over {
  width: 600px;
  z-index: 10;
  position: relative;
}

Now, the z-index of the #under element will work as expected, placing it behind the #over element.