title: How to make a div float in and scroll with the center of page?
body: <p>So, I work on a Facebook FBML App. What I want is simple.  Just have a div shows in the center of the page, and scroll with the page. i.e. always in the center.</p>
  
  <p>It would be easy with normal JS.  I just use the pageYOffset </p>
  
  <p>However, in Facebook using FBJS, I am not sure what I should use.  It doesn't have getPageYOffset().. and I tried getScrollTop().. it doesn't seem the right thing.</p>
  
  <p>So, anyone knows how?</p>