title: Show hidden element when scroll on custom div
body: <p>I have a page like this:</p>
  
  <pre><code>----------------------
  .header
  ----------------------
  
  
  .content
     .hidden-element (fixed right side)
  
  
  ----------------------
  .footer
  ----------------------
  </code></pre>
  
  <p>I want to show the hidden element when you only scroll on content section.</p>
  
  <p>For example you start to scroll down from header section. When you come to content section, hidden element will appear. If you scroll down again and come to footer section, hidden element will be hidden again.</p>
  
  <p>How can I do that?</p>