title: ContentDialog in UWP, bad scrolling behavior
body: <p>I've created this <code>ContentDialog</code> with a tall StackPanel on purpose, but even though the contents are inside a ScrollViewer, the content appears clipped when it doesn't have enough space. What's going on there?</p>
  
  <pre><code>&lt;ContentDialog x:Name="ContentDialog"&gt;
      &lt;ScrollViewer&gt;
          &lt;StackPanel&gt;
              &lt;Button&gt;Hola&lt;/Button&gt;
              &lt;Button&gt;Hola&lt;/Button&gt;
              &lt;Button&gt;Hola&lt;/Button&gt;
              &lt;Button&gt;Hola&lt;/Button&gt;
              &lt;Button&gt;Hola&lt;/Button&gt;
              &lt;Button&gt;Hola&lt;/Button&gt;
              &lt;Button&gt;Hola&lt;/Button&gt;
              &lt;Button&gt;Hola&lt;/Button&gt;
              &lt;Button&gt;Hola&lt;/Button&gt;
              &lt;Button&gt;Hola&lt;/Button&gt;
              &lt;Button&gt;Hola&lt;/Button&gt;
              &lt;Button&gt;Hola&lt;/Button&gt;
          &lt;/StackPanel&gt;
      &lt;/ScrollViewer&gt;
  &lt;/ContentDialog&gt;
  </code></pre>
  
  <p>However, the same content inside a Page shows the ScrollBars properly.</p>