Stopping flexbox from collapsing child width when auto margins are set

Question

I'm using flexbox to create a sticky footer.

body {
    display: flex;
    flex-direction: column;
    min-height: 100vh;
    margin: 0;
}

header, footer {
    background-color: #c9c9c9;
    padding: 20px;
}

main {
    flex: 1;
    margin: 0 auto;
    max-width: 300px;
    border: 1px solid red;
}

<header></header>
<main>This is some content</main>
<footer></footer>

JsFiddle

This works okay, but the width of <main> now collapses to fit the content, rather than expand to the max-width. This only happens when auto margins are set.

Is there any way to make <main> expand to the max-width when auto-margins are set?

Answer 1

Adding width: 100%; to <main> seems to fix this.

Fiddle link.

*, *:before, *:after {
    box-sizing: border-box;  
}

body {
    display: flex;
    flex-direction: column;
    min-height: 100vh;
    margin: 0;
}

header, footer {
    background-color: #c9c9c9;
    padding: 20px;
}

main {
    flex: 1;
    margin: 0 auto;
    max-width: 300px;
    width: 100%;
    border: 1px solid red;
}

<header></header>
<main>This is some content</main>
<footer></footer>