title: Flexbox not full width
body: <p>I have a basic flexbox like this..</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>.masonry_container {
    display: flex;
  }
  
  .masonry_left_col {
    border: 1px solid grey;
  }
  
  .masonry_right_col {
    border: 1px solid grey;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="masonry_container"&gt;
  
    &lt;div class="masonry_left_col"&gt;
      Content
    &lt;/div&gt;
  
    &lt;div class="masonry_right_col"&gt;
      Content
    &lt;/div&gt;
  
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>Why does it not extend to the full width?</p>
  
  <p>I know this is probably really simple but i can't work it out, where am I going wrong?</p>