title: page-break-after not working in flexboxes
body: <p>This doesn't produce the expected result inside print preview in Firefox:</p>
  
  <pre><code>&lt;aside&gt;
    side
  &lt;/aside&gt;
  
  &lt;div&gt;
    &lt;p&gt; page 1 &lt;/p&gt;
    &lt;p&gt; page 2 &lt;/p&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>CSS:</p>
  
  <pre><code>body{
    display: flex;
  }
  
  aside{
    flex: none;
    width: 100px;
  }
  
  div{
    flex: auto;
  }
  
  p{
    break-after: always;
    page-break-after: always;
  }
  </code></pre>
  
  <p>In Chrome and IE I get 2 pages like I should. It appears that FF doesn't break the div in 2 pages when an ancestor is a flex box. Why?</p>