http://jsfiddle.net/XW9Se/
I\'ve set width: 200px;
on the left
The answer from Adrift is perfect; but a change to make it more flex would be
#left{
flex-basis: 200px;
flex-grow: 0;
flex-shrink: 0;
}
And remove the width property entirely.
It would be the flex way to say that the element will have an invariable width of 200px
Also (if nothing from above works)
Check your min-width and max-width. It fixed the same problem for me by increasing their range.