You need to add min-width:0 on .middleColumn, if you want to allow it to shrink below its min-content width (the intrinsic width of its <img>-child).
Otherwise, it gets the new default min-width:auto, which on a flex item will basically make it refuse to shrink below its shrinkwrapped size.
(Chrome hasn't implemented min-width:auto yet. I'm told IE has, in their next-gen rendering engine, so I'd expect that version should behave like Firefox here -- as will Chrome, once they implement this feature.)
Snippet with that fixed:
<!DOCTYPE html>
<html>
<head>
<style type="text/css">
.container {
width:300px;
}
.flexbox {
display:flex;
}
.flexbox .column {
flex:1;
background-color: red;
}
.flexbox .middleColumn {
flex:3;
min-width:0;
}
.flexbox .middleColumn img {
width:auto;
height:auto;
max-width:100%;
max-height:100%;
align-self: center;
display: block;
}
</style>
</head>
<body>
<div class="container">
<div class="flexbox">
<div class="column">This is the left column!</div>
<div class="middleColumn">
<img src="http://placehold.it/400/333333">
</div>
<div class="column">This is the right column!</div>
</div>
</div>
</body>
</html>
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