title: How to set a div to use certain percentage of the height, while using overflow:auto to put a scrollbar?
body: <p>I have the following code:</p>
  
  <pre><code>div.leftnav {
      overflow: auto ;
      height: 50% ;
  }
  </code></pre>
  
  <p>That, when I set a big div of this class, does not work (it doesn't show the scrollbar nor resize it to 50%, however it works perfectly if instead if I use</p>
  
  <pre><code>height: 400px ;
  </code></pre>
  
  <p>Or some other "absolute" value.</p>