title: element.scrollTop always returns 0
body: <p>I'm trying to get the scrollTop position of an element, but it always returns 0. What am I doing wrong, and how can I fix it?</p>
  <p><a href="https://jsfiddle.net/nhg6uedj/" rel="noreferrer"><h3>JSFiddle</h3></a></p>
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>var inner = document.getElementById('inner');
  
  window.addEventListener('scroll', function() {
    console.log(inner.scrollTop);
  })</code></pre>
  <pre class="snippet-code-css lang-css prettyprint-override"><code>#outer {
    background-color: tan;
    height: 1000px;
  }
  #first {
    background-color: bisque;
    height: 200px;
  }
  #inner {
    background-color: aquamarine;
    height: 100px;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div id="outer"&gt;
    &lt;div id="first"&gt;&lt;/div&gt;
    &lt;div id="inner"&gt;scrollTop always returns 0&lt;/div&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>