The answer given by Nico O is correct. However this doesn't get the desired result on Internet Explorer 10 to 11 and Firefox.
For IE, I found that changing
.flex > div
{
flex: 1 0 50%;
}
to
.flex > div
{
flex: 1 0 45%;
}
seems to do the trick. Don't ask me why, I haven't gone any further into this but it might have something to do with how IE renders the border-box or something.
In the case of Firefox I solved it by adding
display: inline-block;
to the items.