I'd love some clarity on the concept of stacking contexts

Question

I'd love some clarity on the concept of stacking contexts

After encountering a bug with a non-clickable video player inside a jquery ui dialog, I resolved the issue by changing position:relative; to position:inherit;

Other solutions included removing position:relative; altogether or adjusting the z-index of the player class to something other than 1.

Upon further reading, it became clear that these changes impacted the stacking context and ultimately fixed the problem. However, I still lack a full understanding of what was happening in my specific scenario or how stacking contexts function in general. Can anyone provide additional examples or suggestions to shed light on this?

<div class="player"> 
    <div id="videoPlayer_wrapper" style=" position: relative; width:580px; height: 192px;">
        <object type="application/x-shockwave-flash" data="/flash/player.swf" width="100%" height="100%" bgcolor="#000000" id="videoPlayer" name="videoPlayer" tabindex="0">
        </object>
    </div> 
</div>

The CSS for player is as follows:

.player {
    margin-bottom: 20px;
    position: relative;
    z-index: 1;
}

css z-index

Answer 1

Answer №1

Phillip Walton's article on stacking contexts was incredibly insightful and helped me immensely in understanding the concept. I embarked on a debugging journey of my own issue which led me to explore this topic further.

It's worth noting that the pink square with z-index: 100; is positioned below the light blue square with z-index: 1; due to its placement within a stacking context created by the .A class using the transform property.

If you'd like to experiment with this yourself, check out this jsbin link which provides an easier platform for testing than inline code on SO: https://jsbin.com/lataga/2/edit?css,output

div {
  width: 200px;
  height: 200px;
  padding: 1rem;
}

.A {
  position: absolute;
  background-color: red;

  /*
    Adding a transform here creates a
    new stacking context for the children of .A
    */
  transform: translateX(0);

  /*    
    Other properties like opacity < 1 can also trigger creation of stacking context
    */
  /*    opacity: 0.99; */

  /*
    Uncomment the following z-index value to see how raising .A affects child elements.
    */
  /*    z-index: 3; */
}

.a {
  position: relative;

  /*
    Even a much higher z-index can't lift .a above .b when constrained within a lower stacking context
    */
  z-index: 100;

  margin-left: 125px;
  background-color: pink;
}

.B {
  position: absolute;
  margin-top: 75px;
  /*    z-index: 2; */
  background-color: blue;
}

.b {
  margin-left: 50px;
  background-color: lightblue;

  /*
    Explicitly specifying z-index, even though not necessary due to natural stacking order rules
    */
  z-index: 1;
}

<div class="A">
  A: 1
  <div class="a">a: 1.100</div>
</div>
<div class="B">
  B: 2
  <div class="b">b: 2.1</div>
</div>

Understanding Stacking Contexts

When positioning an HTML element or assigning a z-index value, a stacking context is created (also happens when non-full opacity is set).

Stacking contexts can be nested within each other, forming a hierarchy of stacking contexts.

Each stacking context operates independently: only descendant elements are part of the stacking process.

Once stacked, the entire element is considered within the parent stacking context's ordering.

Note: The stacking context hierarchy is a subset of the HTML elements' hierarchy as only specific elements create their stacking contexts. Elements without their stacking contexts are integrated into the parent stacking context.

Find more information here

In Simple Terms:

Every stacking context originates from a single HTML element. Once established, it organizes all child elements within a particular section of the stacking order. This means that an element contained in a low-level stacking context cannot appear in front of another element in a higher-level stacking context, irrespective of any z-index values!

...

[...] Besides opacity, several modern CSS properties also form stacking contexts. These include transformations, filters, CSS regions, paged media, and potentially others. Essentially, if a CSS property necessitates offscreen rendering, it must initiate a new stacking context.

More insights here

Answer 2

Phillip Walton's article on stacking contexts was incredibly insightful and helped me immensely in understanding the concept. I embarked on a debugging journey of my own issue which led me to explore this topic further.

It's worth noting that the pink square with z-index: 100; is positioned below the light blue square with z-index: 1; due to its placement within a stacking context created by the .A class using the transform property.

If you'd like to experiment with this yourself, check out this jsbin link which provides an easier platform for testing than inline code on SO: https://jsbin.com/lataga/2/edit?css,output

div {
  width: 200px;
  height: 200px;
  padding: 1rem;
}

.A {
  position: absolute;
  background-color: red;

  /*
    Adding a transform here creates a
    new stacking context for the children of .A
    */
  transform: translateX(0);

  /*    
    Other properties like opacity < 1 can also trigger creation of stacking context
    */
  /*    opacity: 0.99; */

  /*
    Uncomment the following z-index value to see how raising .A affects child elements.
    */
  /*    z-index: 3; */
}

.a {
  position: relative;

  /*
    Even a much higher z-index can't lift .a above .b when constrained within a lower stacking context
    */
  z-index: 100;

  margin-left: 125px;
  background-color: pink;
}

.B {
  position: absolute;
  margin-top: 75px;
  /*    z-index: 2; */
  background-color: blue;
}

.b {
  margin-left: 50px;
  background-color: lightblue;

  /*
    Explicitly specifying z-index, even though not necessary due to natural stacking order rules
    */
  z-index: 1;
}

<div class="A">
  A: 1
  <div class="a">a: 1.100</div>
</div>
<div class="B">
  B: 2
  <div class="b">b: 2.1</div>
</div>

Understanding Stacking Contexts

When positioning an HTML element or assigning a z-index value, a stacking context is created (also happens when non-full opacity is set).

Stacking contexts can be nested within each other, forming a hierarchy of stacking contexts.

Each stacking context operates independently: only descendant elements are part of the stacking process.

Once stacked, the entire element is considered within the parent stacking context's ordering.

Note: The stacking context hierarchy is a subset of the HTML elements' hierarchy as only specific elements create their stacking contexts. Elements without their stacking contexts are integrated into the parent stacking context.

Find more information here

In Simple Terms:

Every stacking context originates from a single HTML element. Once established, it organizes all child elements within a particular section of the stacking order. This means that an element contained in a low-level stacking context cannot appear in front of another element in a higher-level stacking context, irrespective of any z-index values!

...

[...] Besides opacity, several modern CSS properties also form stacking contexts. These include transformations, filters, CSS regions, paged media, and potentially others. Essentially, if a CSS property necessitates offscreen rendering, it must initiate a new stacking context.

More insights here

I'd love some clarity on the concept of stacking contexts

Answer №1

Understanding Stacking Contexts

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