Quick Tip: How z-index and Auto Margins Work in Flexbox

Flexbox is widely used to solve common layout problems such as sticky footers and contour columns. In addition to these features, it offers some other practical features that are less popular. Let's explore two of them!

Key Points

• Flexbox allows the position attribute to be applied to unlocated elements (such as flex projects) even if the static attribute of the flex project is set to z-index. This can be used to control the stacking order of elements.
• Automatic margins in Flexbox can be used to implement custom alignment of flex items along the spindle. They absorb extra space and push adjacent projects away, enabling a unique UI pattern.
• Although it looks similar visually, using automatic margins and flex-grow properties can produce different layout results, especially in terms of the calculated width or height of the element. It is crucial to choose the method that best matches the desired layout.

Flexbox and z-index

You may already know that the z-index attribute is only suitable for positioning elements. By default, the position attribute of all elements is static and is not positioned. When the position attribute of an element is set to relative, absolute, fixed, or sticky, the element is the "positioning" element.

However, unlocated elements (such as flex items) can also receive z-index attributes. The CSS elastic box layout specification points out:

Flex projects are drawn exactly the same way as inline blocks [CSS21], except that the order of the document is modified in order instead of the original document order, and even if position is static, the value of z-index (not auto) also creates a stacking context.

To understand this behavior, consider the following example:

CodePen Demo Link

Here, we define two elements: .front element and .back element. .front The element has a child element, a box with the number "1". .front The element itself is absolutely positioned. Specifically, its position attribute is fixed and covers the entire viewport.

Our .back element is a flex container. It contains two child elements - a box with the numbers "2" and "3". Based on what we discussed above, we can set the z-index attribute of its flex project, even if they are not positioning elements (i.e. their position attribute is static).

Note that when we add z-index: 2 to the flex project by clicking the button in the demo above, they are on top of the .front element.

Flexbox and automatic margins

We can solve common UI patterns by applying automatic margins to flex projects. First, let's assume we want to build this typical title layout:

To build it, we will use flexbox. No need for floating, fixed width or anything like that.

This is our mark:

<header>
  <nav>
    <h1 id="LOGO">LOGO</h1>
    <ul class="menu">
      <li><a href="https://www.php.cn/link/93ac0c50dd620dc7b88e5fe05c70e15b">About</a></li>
      <li><a href="https://www.php.cn/link/93ac0c50dd620dc7b88e5fe05c70e15b">Projects</a></li>
      <li><a href="https://www.php.cn/link/93ac0c50dd620dc7b88e5fe05c70e15b">Contact</a></li>
    </ul>
    <ul class="social">
      <li><a href="https://www.php.cn/link/93ac0c50dd620dc7b88e5fe05c70e15b">Facebook</a></li>
      <li><a href="https://www.php.cn/link/93ac0c50dd620dc7b88e5fe05c70e15b">Twitter</a></li>
    </ul>
  </nav>
</header>

Our CSS is as follows:

header {
  background: https://www.php.cn/link/93ac0c50dd620dc7b88e5fe05c70e15b333;
}

nav {
  display: flex;
  align-items: center;
  width: 90%;
  max-width: 1200px;
  margin: 0 auto;
}

.menu {
  margin-left: 60px;
  margin-right: auto;
}

In this example, our nav element is the flex container, and the logo, main menu and social menu are the flex items. As can be seen from the previous visualization, the first two flex items are aligned along the main axis to the left side of the flex container. Instead, the social menu aligns along the main axis with the right edge of its parent element.

One way to achieve this custom alignment is to add margin-right: auto to the main menu. With just one line of code, we can override the default alignment of the social menu and push it completely to the right of its container. Similarly, we use the align-self property to override the default alignment of the flex item along the cross axis.

In addition to automatic margins, we can also use a second method to build the required layout. First, we remove the margin-right property from the main menu and add flex-grow: 1 to it.

Even if the results look the same in both cases, there is a big difference. With the first solution, our menu has its initial computed width. For example, when the viewport width is 1100px, the menu width will look like this:

On the other hand, with the second solution, the menu width will get larger because we specified flex-grow: 1. When the viewport width is 1100px, its corresponding width is as follows:

CodePen Demo Link

Let's now assume we want to modify the title layout. Here is the new required layout:

The

mark remains the same. We just need to make some changes in CSS:

nav {
  background: https://www.php.cn/link/93ac0c50dd620dc7b88e5fe05c70e15b333;
  display: flex;
  flex-direction: column;
  height: 100vh;
  width: 180px;
  padding: 20px;
  box-sizing: border-box;
}

.menu {
  margin-top: 60px;
  margin-bottom: auto;
}

In this example, note that the social menu aligns with the bottom edge of its parent element. Again, this is done by adding margin-bottom: auto to the main menu. Of course, we can also use flex-grow: 1, but this will increase the height of the menu.

CodePen Demo Link

Another thing to note is that if we define the

property in any example, we will not see any visual differences. This happens because we use automatic margins to align flex items. The justify-content attribute will only take effect when we delete the automatic margin. According to the specification: justify-content

If free space is assigned to automatic margins, the alignment property will not work in that dimension, as the margins will take up space in all the free space left after flex.

Next, let's create a new variant of the title:

This can be easily achieved by setting

to a flex container, without a doubt. However, again, we were able to use automatic margins to produce the same layout. All we have to do is apply justify-content: space-between to the main menu. margin: 0 auto

CodePen Demo Link

Conclusion

In this article, we introduce two little-known flexbox tips. Before we finish, let's review:

Even if the
• property of the flex project is position, we can apply the static attribute to the flex project. z-index
We can use automatic margins to achieve custom alignment of flex items along the spindle.

If you use any of these tips in your project, let us know in the comments below.

FAQ for Z-Index and Automatic Margins in Flexbox

What is Z-Index in Flexbox? How does it work?

is a CSS attribute that controls the vertical stacking order of overlapping elements. In Flexbox, the Z-Index property can be used to control the order in which flex items are along the z-axis. It is important to note that Z-Index is only suitable for positioning elements. The default value is Z-Index, which means the stacking order is equal to its parent element. If a positive number is assigned to auto, the element will be above the parent element. Z-Index

Why does my Z-Index not work in Flexbox?

Your

doesn't work in Flexbox for several reasons. A common reason is that the Z-Index attribute is only suitable for positioning elements. If your element is not positioned (i.e., its Z-Index value is not position, relative or absolute), then fixed will have no effect. Another reason may be that the parent element has a Z-Index value set, which affects the stacking order of the child elements. Z-Index

How does automatic margins work in Flexbox?

In Flexbox, automatic margins have special features. They can absorb additional space and push adjacent projects away. When you set an automatic margin on a flex project, it takes up any remaining space along the spindle, effectively pushing other projects away. This is very useful for aligning items within a flex container.

Can I use Z-Index to control the order of flex items?

Yes, you can use the Z-Index property to control the order of flex items along the z-axis. However, remember that Z-Index is only suitable for positioning elements. If your flex project is not positioned, Z-Index will have no effect.

Why is my flex project not properly aligned with automatic margins?

If your flex project is not properly aligned with automatic margins, it may be due to several reasons. A common reason is that the flex container has a fixed height or width, which limits the space available to the margins. Another reason might be that the flex project has a fixed size, which prevents the margin from absorbing extra space.

How to solve the Z-Index problem in Flexbox?

Solve the Z-Index problem in Flexbox usually involves making sure your element is positioned and the Z-Index value is set correctly. If your Z-Index does not work, check if your element is positioned. If not, you can locate it by setting the relative, absolute or fixed values. Also, check the position values ​​of the parent element, as they affect the stacking order of the child elements. Z-Index

Can I use automatic margins to center the flex item?

Yes, you can use automatic margins to center flex items in the flex container. By setting automatic margins on all sides of the flex project, it will be centered vertically and horizontally within the flex container.

What is the default value of Z-Index in Flexbox?

The default value of

in Z-Index in Flexbox is auto. This means that the stacking order of the flex item is equal to its parent element. If you want to change the stacking order, you can assign positive or negative numbers to Z-Index.

How does Z-Index affect the stacking order of elements?

Z-Index Influence the stacking order of elements by determining which elements appear on top of other elements. Elements with higher Z-Index values ​​will appear above elements with lower Z-Index values. If two elements have the same Z-Index, the elements that appear after HTML will appear at the top.

Can I use Z-Index with Flexbox to create overlapping elements?

Yes, you can use the Z-Index attribute with Flexbox to create overlapping elements. By assigning different Z-Index values ​​to your flex project, you can control which items appear on top of other projects, creating overlapping effects. Remember that Z-Index is only suitable for positioning elements, so you need to position your flex project for Z-Index to take effect.

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