Margin collapsing in flexbox

Question

Typically, in CSS, when a parent and its last child have a margin, the margins collapse to create a single margin. E.g.

As you can see, even though a margin of 20px is specified on both the article and the main tags, you only get a 20px margin between the last article and footer.

When using flexbox, however, we get a 40px margin between the last article and footer — a full 20px margin from the article to main, and another 20px from main to footer.

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

article {
  margin-bottom: 20px
}

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

footer {
  background: skyblue;
}

<div id="container">
  <main>
    <article>
      Item 1
    </article>
    <article>
      Item 2
    </article>
  </main>
  <footer>Footer</footer>
</div>

Is there a way to make flexbox margins behave the same way as the non-flexbox ones?

Answer 1

Margin collapsing is a feature of a block formatting context.

There is no margin collapsing in a flex formatting context.

3. Flex Containers: the flex and inline-flex display values

A flex container establishes a new flex formatting context for its contents. This is the same as establishing a block formatting context, except that flex layout is used instead of block layout. For example, floats do not intrude into the flex container, and the flex container’s margins do not collapse with the margins of its contents.