Why Do Percentage Paddings Disrupt Flexbox Layout?

Why Percentage Paddings Break Flex Item Behavior

Flexbox layout is designed to distribute items evenly along an axis. However, when applying percentage paddings to flex items (non-flex elements contained within a flex container), the container's layout can become disrupted.

In this example, we have a flex container (<ul>) containing several non-flex items (<li>). When percentage paddings are applied to the individual <li> elements, the container is split into multiple lines:

li {
  display: inline-block;
  padding: 0 5%;
  border: 1px solid black;
}

header {
  display: flex;
}

ul {
  border:1px solid red;
}

However, when fixed paddings are used (e.g., padding: 0 30px), the container remains in a single line.

Explanation:

The behavior of percentage paddings is due to the calculation process involved. The padding is determined relative to the width of the containing block. This width is initially unknown and must be calculated based on the content or any fixed width values.

In the case of percentage paddings, the width calculation occurs after the padding is applied. Therefore, the container's width is adjusted taking into account the padding, leading to an incorrect calculation. This results in the uneven distribution of items and the breaking of the flex container's layout.

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