title: Set Focus for TextBox in Style Trigger
body: <p>I have the following but the Trigger is not working. It is supposed to place cursor in the <code>TextBox</code> ready to type.</p>
  
  <p>Can someone point me in the right direction please.</p>
  
  <pre><code>&lt;Popup Name="PopupGoto" Placement="Mouse" StaysOpen="False" 
         IsOpen="{Binding Path=GotoPopupIsOpen, Mode=OneWay, UpdateSourceTrigger=PropertyChanged}"&gt;
  
      &lt;Popup.Style&gt;
          &lt;Style TargetType="Popup"&gt;
              &lt;Style.Triggers&gt;
                  &lt;Trigger Property="IsOpen" Value="True"&gt;
                      &lt;Setter Property="FocusManager.FocusedElement" Value="{Binding ElementName=GotoTextbox}" /&gt;
                  &lt;/Trigger&gt;
              &lt;/Style.Triggers&gt;
          &lt;/Style&gt;
      &lt;/Popup.Style&gt;
  
      &lt;Border&gt;
          &lt;Grid&gt;
              &lt;TextBox x:Name="GotoTextbox" Text="{Binding GotoLineNumber}" /&gt;
              &lt;Button Content="Goto" Command="{Binding GotoCommand}" /&gt;
          &lt;/Grid&gt;
      &lt;/Border&gt;
  &lt;/Popup&gt;
  </code></pre>