title: TreeViewItem not showing its expander control
body: <p>I have the following XAML:</p>
  
  <pre><code>&lt;TreeView&gt;
     &lt;TreeViewItem ItemsSource={Binding} Header="TopMost" IsExpanded="True"&gt;
        &lt;TreeViewItem.ItemTemplate&gt;
           &lt;DataTemplate&gt;
              &lt;TreeViewItem&gt;
                &lt;TreeViewItem.Header&gt;
                  &lt;TextBlock Text="{Binding SubTopic}"/&gt;
                &lt;TreeViewItem.Header&gt;
                &lt;!-- further data representation --&gt;
              &lt;/TreeViewItem&gt;
          &lt;/DataTemplate&gt;
        &lt;/TreeViewItem.ItemTemplate&gt;
     &lt;/TreeViewItem&gt;
     &lt;TreeViewItem&gt;
     &lt;/TreeViewItem&gt;
  &lt;/TreeView&gt;
  </code></pre>
  
  <p>The first TreeViewItem does expand and show its children, but the selector that should be visible on the left of the text TopMost is not present. I can get the children of the main TreeViewItem to collaps and expand by double clicking the TopMost Header. How do I solve this?</p>