title: Xaml designer doesn't show content of a usercontrol
body: <p>We have a specialized TabItem defined in xaml:</p>
  
  <pre><code>&lt;TabItem  x:Class="MyApp.MyViewTab" xmlns:my="clr-namespace:MyApp"&gt;
      &lt;my:MyView /&gt;
  &lt;/TabItem&gt;
  </code></pre>
  
  <p>and code behind: <code>public class MyViewTab : TabItem</code>.
  And <code>MyView</code> is a <code>UserControl</code>:   </p>
  
  <p><code>MyView</code> is correctly shown in the xaml designer and also at run time correctly displayed as the content of the TabItem.  </p>
  
  <p>Why does the xaml designer not display the content of <code>MyView</code>? It is a gray rectangle, top half light-gray, bottom half darker-gray.</p>