title: Double scrollbars
body: <p>I have this <strong><a href="https://jsfiddle.net/qrsotzpr/2/" rel="nofollow noreferrer">JSFIDDLE</a></strong> with a parallax effect on my images. 
  The problem is that I have two scrollbars in the result. 
  How can I have only one scrollbar for all my content ?</p>
  
  <p>HTML : </p>
  
  <pre><code>&lt;div id="intro"&gt;
  &lt;p&gt;Sed uelit, sed quia...&lt;/p&gt;
  &lt;/div&gt;
  
  &lt;div id="text"&gt;
  &lt;h1&gt;Hello&lt;/h1&gt;
  &lt;/div&gt;
  &lt;div id="wrapper"&gt;
      &lt;section id="frame1"&gt;
          &lt;div class="back"&gt;
          &lt;/div&gt;
      &lt;/section&gt;
  
  
          &lt;section id="frame7"&gt;
          &lt;div class="back"&gt;
  
          &lt;/div&gt;
      &lt;/section&gt;
  
  &lt;/div&gt;
  </code></pre>