Why Do Flexbox Margins Behave Differently Than in a Standard Block Formatting Context?

Margin Collapsing in Flexbox

Typically, in CSS, when a parent and its last child have a margin, the margins collapse into a single margin. However, in flexbox, margins behave differently.

Why Margin Collapse is Different in Flexbox

Margin collapsing is a feature of a block formatting context, which is not used in a flex formatting context. Flexbox establishes a new flex formatting context for its contents, meaning margins do not collapse in the same way as in a block formatting context.

How to Make Flexbox Margins Behave Like Non-Flexbox Ones

Since margin collapsing is a feature of a block formatting context, it is not possible to make flexbox margins behave exactly like non-flexbox ones. However, it is possible to achieve a similar effect by setting the flexbox container's display property to inline-flex instead of flex. This will create a flexbox formatting context, but it will be treated as an inline element, and thus the margins will collapse as expected.

For example:

#container {
  display: inline-flex;
  flex-direction: column;
}

article {
  margin-bottom: 20px;
}

main {
  background: pink;
  margin-bottom: 20px;
}

footer {
  background: skyblue;
}

In this case, the margins will collapse between the last article and the footer, resulting in a 20px margin, just like in a non-flexbox layout.

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