title: CSS overlay div not stretching to document height
body: <p>I have an overlay element that gets appended to the document with JavaScript. Here is its style declarations:</p>
  
  <pre><code>#overlay_mask {
      display: none;
      position: absolute; 
      top: 0; 
      left: 0; 
      z-index: 9999;
      width: 100%;
      height: 100%;
      background-color: #000000;
  }
  </code></pre>
  
  <p>It's size stretches to the height of the window correctly, but when the user scrolls, the overlay doesn't follow.</p>
  
  <p>The <code>html</code> and <code>body</code> elements are set to <code>width: 100%</code>; and <code>height: 100%</code>. The overlay is the first child of the document.</p>