title: How to position: fixed the navbar when using flexbox
body: <p>Starting with this layout: 
  <a href="http://codepen.io/RebelOne/pen/wodbpR" rel="noreferrer">http://codepen.io/RebelOne/pen/wodbpR</a></p>
  
  <pre><code>.navbar {
    display: flex;
    justify-content: space-between;
  </code></pre>
  
  <p>When I add position: fixed to the navbar, it collapses down and doesn't fill the width of the wrapper.  Does anyone know how to fix this?
  <a href="http://codepen.io/RebelOne/pen/bBWyLZ?editors=1100" rel="noreferrer">http://codepen.io/RebelOne/pen/bBWyLZ?editors=1100</a></p>
  
  <pre><code>.navbar {
    display: flex;
    justify-content: space-between;
    background-color: yellow;
    position: fixed;
    top: 0;
    z-index: 99;
    }
  </code></pre>