Why Are My Flex Items Ignoring Margins When Using `box-sizing: border-box`?

Flex Items Ignoring Margins with box-sizing: border-box

Flexbox can be tricky when it comes to margins and box sizing. By default, margins are not included in the calculation of the flex item's size, even when using box-sizing: border-box.

Understanding the Box Model

The box model is a CSS concept that defines the dimensions and layout of elements. It consists of four sections:

• Content box: The size of the actual content of the element.
• Padding: Transparent space inside the border.
• Border: Visible line around the element.
• Margin: Transparent space outside the border.

box-sizing

The box-sizing property determines how padding and borders are calculated into an element's overall size. It has two possible values:

• content-box (default): Padding and borders are added to the content box.
• border-box: Padding and borders are included in the element's size.

Flexbox Properties

In flexbox, the following properties relate to the layout of flex items:

• flex-grow: Specifies the amount of extra space an item should take up when there is free space.
• flex-shrink: Specifies the amount an item should shrink in order to fit within the container.
• flex-basis: Defines an ideal size of an item before applying flex-grow or flex-shrink.

Solution

To ensure flex items respect margins while box-sizing: border-box is used, consider the following:

• Specify flex-shrink: 0 on flex items to prevent them from shrinking.
• Adjust the flex-basis value to account for both the content size and margins.

For example, in your code:

header>span {
  flex: 1 0 26%; /* Adjusted from 1 1 33.33% */
  margin: 10px;
}

By adjusting the flex-basis, we ensure there is enough space for both the content and the margins.

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