Collapsing margin on Flexbox children

Question

I have the following arrangement via flexbox with flex-wrap and elements able to stretch using flex-grow:

Each item has a margin on all sides. This is to separate the items from each other, but the side effect is the whole block has margins which I'd like to collapse. It could be done with rules like nth-child(-n+3) { margin-top: 0; } but because the container size could vary, there could be any number of items per row and any number of rows. So I'm wondering if flex-box has any way to collapse the outer margins in a setup like this, while retaining the margins between items.

JSBin

The HTML is simply 6 items inside a container.

The CSS (Sass) is as follows:

.container
  display: flex
  flex-wrap: wrap
  background: #eef
  align-items: stretch

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

Answer 1

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