title: Nested ListBox scrolling issue
body: <p>I have a listbox within listbox and when I try to reduce the height of the window the scrollbar for the listbox does not appears up. I want to dispaly the scrollbar on the outer listbox because the outerlistbox can have multiple innner listboxes.
  here is the XAML I am using. Can anybody let me know what is the issue?</p>
  
  <pre><code>&lt;ListBox Name="MasterListBox" HorizontalAlignment="Stretch" Width="200"&gt;
      &lt;ListBoxItem &gt;
          &lt;ListBox Name="Child1" BorderBrush="{x:Null}"&gt;
              &lt;ListBoxItem Content="testing item1" /&gt;
              &lt;ListBoxItem Content="testing item2" /&gt;
              &lt;ListBoxItem Content="testing item3" /&gt;
              &lt;ListBoxItem Content="testing item1" /&gt;
              &lt;ListBoxItem Content="testing item2" /&gt;
              &lt;ListBoxItem Content="testing item3" /&gt;
              &lt;ListBoxItem Content="testing item1" /&gt;
              &lt;ListBoxItem Content="testing item2" /&gt;
              &lt;ListBoxItem Content="testing item3" /&gt;
              &lt;ListBoxItem Content="testing item1" /&gt;
              &lt;ListBoxItem Content="testing item2" /&gt;
              &lt;ListBoxItem Content="testing item3" /&gt;
              &lt;ListBoxItem Content="testing item1" /&gt;
              &lt;ListBoxItem Content="testing item2" /&gt;
              &lt;ListBoxItem Content="testing item3" /&gt;
              &lt;ListBoxItem Content="testing item1" /&gt;
              &lt;ListBoxItem Content="testing item2" /&gt;
              &lt;ListBoxItem Content="testing item3" /&gt;
          &lt;/ListBox&gt;
      &lt;/ListBoxItem&gt;
  
  &lt;/ListBox&gt;
  </code></pre>