title: Show div after scroll and hide when scrolling back up
body: <p>I have a div which needs to be shown when a user scrolls 400 pixels down. When he scroll back up, above the 400 pixels, this div needs to be hidden again. I can make this work with <code>fadeIn()</code> and <code>fadeOut(</code>) but in this case I want to add a small animation similar to <code>slideUp()</code> by using the animate(). But doing it this way makes it very buggy, sometimes it will appear, other times not.</p>
  
  <p>Why is this?</p>
  
  <p><a href="http://jsfiddle.net/3k28r/" rel="nofollow">Demo</a></p>