title: Changing the TreeView ItemsPanel orientation has no effect
body: <p>The default <code>ItemsPanel</code> of a <code>TreeView</code> is a <code>StackPanel</code> that has the default orientation <code>Vertical</code>. I have changed the orientation of the <code>StackPanel</code> to <code>Horizontal</code>:</p>
  
  <pre><code>&lt;controls:TreeView&gt;
    &lt;controls:TreeView.ItemsPanel&gt;
      &lt;ItemsPanelTemplate&gt;
        &lt;StackPanel Orientation="Horizontal"/&gt;
      &lt;/ItemsPanelTemplate&gt;
    &lt;/controls:TreeView.ItemsPanel&gt;
    &lt;controls:TreeViewItem Header="Root" IsExpanded="True"&gt;
      &lt;controls:TreeViewItem Header="Alfa"/&gt;
      &lt;controls:TreeViewItem Header="Beta"/&gt;
      &lt;controls:TreeViewItem Header="Gamma"/&gt;
    &lt;/controls:TreeViewItem&gt;
  &lt;/controls:TreeView&gt;
  </code></pre>
  
  <p>I would expect the subordinate items in the tree to stack horizontally instead of vertically, but I see no visible change when I swap the orientation from <code>Vertical</code> to <code>Horizontal</code> and back again. Why doesn't the <code>Orientation</code> of the <code>StackPanel</code> affect the layout of the items in the <code>ItemsPresenter</code>?</p>