Issue when centering vertically with flexbox when heights are unknown

Question

My layout has a container flex-container and a child.

HTML:

  
text
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Answer 1

I found the solution:

.flex-container {
    display: flex; /* only */
    overflow-y: scroll;
}

.flex-container > div {
    margin: auto; /* horizontal and vertical align */
}

html, body {
    height: 100%;
    width: 100%;
    padding: 0;
    margin: 0;
}

.flex-container {
    display: flex;
    width: 100%;
    height: 100px; /* change height to 300px */
    overflow-y: scroll;
    background: #2a4;
}

.flex-container > div {
    padding: 1em 1.5em;
    margin: auto;
    background: #E77E23;
    width: 400px;
}

  
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Codepen: http://codepen.io/ces/pen/Idklh