title: Calculate scrollbar height according to amount of hidden contents
body: <p>I'm creating a graphical scrollbar so I need to calculate the scrollbar height manually. You know how in most applications the scrollbar height changes based on how much there is to scroll?</p>
  
  <p>What is the formula for calculating the scrollbar height based on the amount of hidden contents? Is it logarithimic or exponential or just plainly based on the percentage of contents visible vs contents hidden?</p>
  
  <p>These are my input variables:</p>
  
  <ul>
  <li>Visible area - eg. 100 px</li>
  <li>Content height - eg. 1000 px</li>
  <li>Max scrollbar height - eg. 500 px</li>
  </ul>
  
  <p>This is what I want to calculate:</p>
  
  <ul>
  <li>Scrollbar height - eg. 50 px??</li>
  </ul>