title: Lost focus event in pop up not working in user control wpf
body: <p>I have a pop up control inside a UserControl:</p>
  
  <pre><code>&lt;UserControl ....&gt;
  &lt;Popup x:Name="popUp" IsOpen="False" AllowsTransparency="True" LostFocus="popUp_LostFocus"&gt;
  ....
  &lt;/Popup&gt;
  &lt;/UserControl&gt;
  </code></pre>
  
  <p>The popup has a LostFocus Event:</p>
  
  <pre><code>private void popUp_LostFocus(object sender, RoutedEventArgs e)
  {
      ...
  }
  </code></pre>
  
  <p>But when I use the user control in a Window and the pop up lost the focus, the event not working.</p>