title: Expand Child When Parent IsExpanded in a TreeView
body: <p>I am attempting to expand a Child Node when its parent is expanded.</p>
  
  <p>Otherwise stated: (Child.IsExpanded == Parent.IsExpanded)</p>
  
  <p>This appears right, but does not seem to work:</p>
  
  <pre><code>&lt;TreeView ItemsSource="{Binding}"&gt;
      &lt;TreeView.ItemContainerStyle&gt;
          &lt;Style TargetType="TreeViewItem"&gt;
              &lt;Style.Triggers&gt;
                  &lt;DataTrigger Value="True" 
                               Binding="{Binding Path=IsExpanded, 
                               RelativeSource={RelativeSource 
                               Mode=FindAncestor, 
                               AncestorType={x:Type TreeViewItem}, 
                               AncestorLevel=2}}"&gt;
                      &lt;Setter Property="IsExpanded" Value="True" /&gt;
                  &lt;/DataTrigger&gt;
              &lt;/Style.Triggers&gt;
          &lt;/Style&gt;
      &lt;/TreeView.ItemContainerStyle&gt;
  &lt;/TreeView&gt;
  </code></pre>
  
  <p>Neither does this:</p>
  
  <pre><code>&lt;TreeView ItemsSource="{Binding}"&gt;
      &lt;TreeView.ItemContainerStyle&gt;
          &lt;Style TargetType="TreeViewItem"&gt;
              &lt;Setter Property="IsExpanded"
                      Value="{Binding Path=IsExpanded, 
                               RelativeSource={RelativeSource 
                               Mode=FindAncestor, 
                               AncestorType={x:Type TreeViewItem}, 
                               AncestorLevel=2}}" /&gt;
          &lt;/Style&gt;
      &lt;/TreeView.ItemContainerStyle&gt;
  &lt;/TreeView&gt;
  </code></pre>
  
  <p>What's missing here?</p>
  
  <p>Thanks in advance.</p>