Collapsing margin on Flexbox children

It's a bit of a hack, but you can add a negative margin on the flex container to cancel out the items' margins along the edges, and then move its "background" styling to a parent wrapper-element.

Updated JSBin

Updated CSS (SASS):

.wrapper
  background: #eef
  border: 1px solid darkgray

.container
  display: flex
  flex-wrap: wrap
  margin: -1em

.item
  flex-grow: 1  
  margin: 1em
  border: 1px solid black
  padding: 1em
  min-width: 6em