title: Test in JQuery if an element is at the top of screen
body: <p>I have a <code>div</code> that is positioned about 100px from the top of the browser window.  When the user scrolls down, I want the <code>div</code> to stay where it is <strong>until</strong> it reaches the top of the screen.  Then, I'll change some CSS with JQuery to make the position to change to fixed and the margin-top to 0.  How can I test in JQuery if this <code>div</code> is at the top of the screen?</p>