title: Install the event filter for QAbstractScrollArea
body: <p>Why we cannot install an event filter for <code>QAbstractScrollArea</code> normal way:</p>
  
  <pre><code>obj-&gt;installEventFilter(this)
  </code></pre>
  
  <p>Instead, we should install event filter on <code>QAbstractScrollArea::viewport()</code>. Why leads to this manner?</p>