title: How to make the middle element stick when scrolled down
body: <p>There are 3 elements in My HTML, like</p>
  
  <pre><code>&lt;div id="summary"&gt;&lt;/div&gt;
  &lt;div id="header"&gt;&lt;/div&gt;
  &lt;ul id="scroll-list"&gt;&lt;/ul&gt;
  </code></pre>
  
  <p>how could i write my css to make sure when i scroll-down the whole page, <code>summary</code> will scroll up, when <code>summary</code> disappear, <code>header</code> will stick on the top?</p>