title: Get div to show/hide when scrolling for a "back to top" link
body: <p>I can't make my "go to top" <code>id=arrow-up</code> div to disappear when on e.g. top of the page.</p>
  
  <p>At the top of the page I got
       </p>
  
  <p>So I would like the <code>arrow-up</code> div to <code>visible(show("slow"))</code> when not on top of the page.</p>
  
  <pre><code> var tmp = $(window).height();
  </code></pre>
  
  <p><code>Tmp</code> is used to get the viewport height... Not sure if that is right?</p>
  
  <p>I have seen other solutions but they are only kind of the same... and I can't make them work, also they font use <code>:in-viewport</code>. Can I make it with viewport or am I side tracked?</p>
  
  <pre><code>&lt;script type="text/javascript" src="http://code.jquery.com/jquery-latest.min.js"&gt;        &lt;/script&gt;
  &lt;script src="bootstrap-3.1.1-dist/js/bootstrap.min.js"&gt;&lt;/script&gt;
  &lt;script src="http://malsup.github.com/jquery.form.js"&gt;&lt;/script&gt;
  &lt;script src="/jquery/isInViewport.min.js"&gt;&lt;/script&gt;
  
  
  
  $(window).scroll(function() {
    if($('#pageStart:in-viewport(tmp)')){
      $("#arrow-up").hide("slow");
    }else{
      $("#arrow-up").show("slow");
    }
  });
  </code></pre>