title: Fixed Navigation that stays within its parent div
body: <p>I'm making a fluid width website, and on one of my pages I have a fixed sidebar.  Right now it works fine as I only have two links on it; The sidebar isn't very tall.  The layout goes like this:</p>
  
  <p>Header</p>
  
  <p>Content with Sidebar</p>
  
  <p>Footer</p>
  
  <p>The sidebar is positioned fairly far down because of the header, and if it gets too tall it will overflow into the footer.  I want it to start and being 'fixed' once it reaches certain heights on the screen</p>
  
  <p>In other words:</p>
  
  <ol>
  <li>Relative until scroll down</li>
  <li>Fixed until reach footer</li>
  <li>Relative once footer is reached</li>
  </ol>