unicorn-utterances/content/blog/css-stacking-context/index.md

title, description, published, authors, tags, attached, license

title	description	published	authors	tags	attached	license
Why is z-index not working?! - Explaining CSS Stacking Context		2023-01-01T22:12:03.284Z	crutchcorn	webdev css html		cc-by-4

Despite some some UX headaches modals can introduce into an app, they're still a widely used UI element in many applications today.

While building sufficiently useful modals can be a challenging task, a rudimentary modal can be completed even without JavaScript.

Let's use some CSS and HTML in order to build a basic modal:

<div>
  <div id="body">
    <p>This is some text, pretend it's an app back here</p>
  </div>
  <div id="modal-container">
    <div id="modal">This is a modal</div>
  </div>
</div>

<style>
#modal-container {
  position: fixed;
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
  display: flex;
  justify-content: center;
  align-items: center;
  background: rgba(0, 0, 0, 0.5);
}

#modal {
  background: white;
  border: 1px solid black;
  padding: 1rem;
  border-radius: 1rem;
}
</style>

Tada! 🎉 Now we have a fairly basic modal to display whatever HTML we want inside.

But let's say that we keep building out the page. As we do, we might, for example, want to have a footer beneath our main page's content.

<div>
  <div id="body" style="min-height: 50vh">
    <p>This is some text, pretend it's an app back here</p>
  </div>
  <div id="modal-container">
    <div id="modal">This is a modal</div>
  </div>
  <footer style="min-height: 50vh">App Name</footer>
</div>

<style>
#modal-container {
  position: fixed;
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
  display: flex;
  justify-content: center;
  align-items: center;
  background: rgba(0, 0, 0, 0.5);
}

#modal {
  background: white;
  border: 1px solid black;
  padding: 1rem;
  border-radius: 1rem;
}

footer {
  position: relative;
  background: lightblue;
  padding: 1rem;
}
</style>

At first glance, this might look like it's been successful, but let's take a look at the rendered output:

Oh dear! Why is the footer rendered above the modal?

Well, my friends, the modal is rendering under the footer due to something called "The Stacking Context".

What is the stacking context?

While the concept of the "Stacking Context" in the DOM is quite complex, here's the gist of it:

While we often think about our browser as displaying a 2-dimensional image as a result of our HTML and CSS, this isn't the case. Take the following code example:

<div id="container">
  <div id="blue">Blue</div>
  <div id="green">Green</div>
  <div id="purple">Purple</div>
</div>

<style>
#container {
  display: relative;
}

#blue,
#green,
#purple {
  height: 100px;
  width: 100px;
  position: absolute;
  padding: 8px;
  color: white;
  border: 4px solid black;
  border-radius: 4px;
}

#blue {
  background: #0f2cbd;
  left: 50px;
  top: 50px;
}

#green {
  background: #007a70;
  left: 100px;
  top: 100px;
}

#purple {
  background: #5f00b2;
  left: 150px;
  top: 150px;
}
</style>

Here, we have three different boxes that overlap on one another. Given that they overlap, which one do you think takes priority and, at least visually, is on top of the other boxes?

No, really, guess! Stop reading, take a look at the code, and take a guess. 😊

Ready to see the answer?

OK, here it is:

While some CSS pros might assume that purple is the priority due to order in which the CSS is laid out, just like other CSS rules, this isn't what's happening here.

Notice how the purple box seemingly remains on "top" when we re-arrange the CSS rules:

#purple {
  background: #5f00b2;
  left: 150px;
  top: 150px;
}

#green {
  background: #007a70;
  left: 100px;
  top: 100px;
}

#blue {
  background: #0f2cbd;
  left: 50px;
  top: 50px;
}

If changing the CSS order doesn't re-arrange the boxes, then what does?

Well...

Re-arrange HTML Elements to Change the Stacking Order

Let's take the HTML we had before, and re-arrange it a bit:

<div id="container">
  <div id="purple">Purple</div>
  <div id="green">Green</div>
  <div id="blue">Blue</div>
</div>

Now if we look at the box order, we'll see...

Now our boxes have reversed their height order! This is because one of the deciding factors of an element's z position is its relationship to other elements.

Positioned Elements Behave Differently Than Non-Positioned Elements

This is where things get confusing. Take your time with this chapter, it's okay to have to re-read this section multiple times.

While we were using absolutely positioned elements for a simple demo before, let's take a step back and change our elements to be positioned using margin instead:

<div id="container">
  <div id="purple">Purple</div>
  <div id="green">Green</div>
  <div id="blue">Blue</div>
</div>

<style>
#container {
  display: relative;
}

#container > div:nth-child(1) {
  margin-top: 50px;
  margin-left: 50px;
}

#container > div:nth-child(2) {
  margin-top: -50px;
  margin-left: 100px;
}

#container > div:nth-child(3) {
  margin-top: -50px;
  margin-left: 150px;
}

#blue,
#green,
#purple {
  height: 100px;
  width: 100px;
  padding: 8px;
  color: white;
  border: 4px solid black;
  border-radius: 4px;
}

#blue {
  background: #0f2cbd;
}

#green {
  background: #007a70;
}

#purple {
  background: #5f00b2;
}
</style>

Looks like a familiar output:

While working on styling, we wanted our green box to move to the left when you hover over it. This is straightforward enough to do using CSS animations, let's add it:

#green {
  background: #007a70;
  position: relative;
  left: 0px;
  transition: left 300ms ease-in-out;
}

#green:hover {
  left: 20px;
}

While our green button now smoothly moves left when you hover over it, there's a new problem: The green box is now on top of the purple and blue boxes.

This is because positioning an element introduces a "stacked context". This means that our relative positioned element takes priority in the z layer over non-positioned elements.

Understanding more rules of Stacked Contexts

While relative positioning is one way that you can take priority in a stacked context, it's far from the only way to do so. Here's a list of CSS rules that will take priority in a stacked context, from the lowest priority to the highest priority:

• Positioned elements with a negative z-index
• The background and borders of the parent element
• Non-positioned elements
• Elements with a float style applied
• Non-positioned inline elements
• Positioned elements without a z-index applied, or with a z-index of 0

So, if we have the following HTML:

<div class="container" style="background: rgba(0, 0, 0, 0.8)">
    <div class="box slate" style="position: relative">Slate</div>
    <div class="box yellow" style="display: inline-block">Yellow</div>
    <div class="box lime" style="float: left">Lime</div>
    <div class="box green" style="">Green</div>
    <div class="box cyan" style="position: relative; z-index: -1">Cyan</div>
</div>

We would see, from top to bottom:

• A slate colored box
• A yellow colored box
• A lime colored box
• A green colored box
• The container's background
• A cyan colored box

All of these rules are superseded by the order of the elements within the HTML, as we learned before. For example, with the following HTML:

<div class="container" style="background: rgba(0, 0, 0, 0.8)">
  <div class="box slate" style="position: relative">Slate</div>
  <div class="box yellow" style="">Yellow</div>
  <div class="box lime" style="position: relative">Lime</div>
  <div class="box cyan" style="">Cyan</div>
</div>

You would see the following order of elements:

• Lime
• Slate
• Cyan
• Yellow

This is because the lime and slate take priority over yellow and cyan thanks to their relative positioning, but are still in HTML order within the same z level priority and within the same stacking context.

Creating Stacking Contexts

"Welp, that's enough reading in the book today"

You think to yourself. You go lay down and get some sleep. In your dreams, you can still hear the book speaking to you:

[...] are still in HTML order within the same z level priority and within the same stacking context

[...] within the same stacking context

The book repeats itself:

[...] within the same stacking context

You wake up, realize that you don't yet know what that sentence means, and think to yourself:

There's no way this gets even more complicated.

Unfortunately, it does.

---

At its heart, a stacking context is a group that you can move multiple items up or down the z axis at the same time.

Take the following HTML:

<div class="container">
  <div id="top-container" style="position: relative">
    <div class="box slate" style="position: relative">Slate</div>
    <div class="box yellow" style="">Yellow</div>
  </div>
  <div id="bottom-container">
    <div class="box lime" style="position: relative">Lime</div>
    <div class="box cyan" style="">Cyan</div>
  </div>
</div>

What order do you think the boxes are going to be in?

The answer is:

• Slate
• Lime
• Cyan
• Yellow

This is because, despite the parent top-container having position: relative, the boxes are still within the same stacking context. This stacking context follows the same ordering rules as outlined before, which means that the positioned slate and lime boxes take z priority over cyan and yellow.

Ready for the twist?

Let's add z-index to our top-container:

<div class="container">
  <div style="position: relative; z-index: 1">
    <div class="box slate" style="position: relative">Slate</div>
    <div class="box yellow" style="">Yellow</div>
  </div>
  <div>
    <div class="box lime" style="position: relative">Lime</div>
    <div class="box cyan" style="">Cyan</div>
  </div>
</div>

Now what order do you think they'll be in?

• Slate
• Yellow
• Lime
• Cyan

This is because, in reality, what we're ordering here is not the boxes, but instead is the top-container and bottom-container divs, then the boxes, like so:

• top-container
  • slate
  • yellow
• bottom-container
  • lime
  • cyan

The reason this only occurred when we added a z-index to top-container is because that's when a new stacking context was created. When that context was created, we raised it to a higher z axis due to the same ordering rules as before.

Remember, a stacking context is a grouping of elements that move together as a collection when the parent's z axis location is changed.

Stacking Contexts are created when:

• z-index is applied to a positioned element

• z-index is applied to a child of a grid or flex element

• Element with an opacity less than 1

• Element with any of the following properties:

  • transform
  • filter
  • backdrop-filter
  • perspective
  • clip-path
  • mask / mask-image / mask-border

This list is non-exhaustive, but contains most of the highlights of when a stacking context is created.

It's worth mentioning that if a stacking context is created, then the element that created said stacking context is treated with priority z axis ordering.

For example, if you have:

<div>
	<div style="position: absolute; top: 0; background: white">Absolute</div>
	<div style="opacity: 0.99; background: white">Opacity</div>
</div>

Then it will show "Absolute" above "Opacity", thanks to the order of the HTML sequence; this is all despite positioned elements typically being prioritized above HTML sequencing.

If we remove the opacity: 0.99 from the "Opacity" div, then "Absolute" will be on top.

Stacking Stacking Contexts

While the previous sections have been head scratchers, let's dive into mind melting territory: You can contain stacking contexts within other stacking contexts. 🤯

// TODO: Write

The Problem with Stacking Contexts

// TODO: Explain that z-index cannot escape

If you want to learn more about the "stacking context", I'd suggest reading through the following resources:

• Stacking elements - CSS z-index and stacking context explained - NetGen
• The stacking context - MDN
• What The Heck, z-index?? - Josh W Comeau
• What No One Told You About Z-Index - Philip Walton
• Appendix E. Elaborate description of Stacking Contexts - W3C