title: WP7 Popup not showing
body: <p>In my app I want to display a simple string within a popup when the user clicks on an image. For this I added a <code>Tap</code> gesture listener to the image and within the handler I have the following code:</p>
  
  <pre><code>private void GestureListener_Tap( object sender, GestureEventArgs e )
  {
    var img = sender as Image;
    if( img == null ) {
      return;
    }
  
    Point pos = e.GetPosition( img );
    string text = "I'm a popup!";
  
    var popup = new Popup() {
      Child = new Border() {
        BorderBrush = new SolidColorBrush( Colors.LightGray ),
        Child = new TextBlock() {
          Text = text,
          TextWrapping = TextWrapping.Wrap,
        },
      },
      HorizontalAlignment = HorizontalAlignment.Stretch,
      HorizontalOffset = pos.X,
      VerticalOffset = pos.Y,
      Visibility = Visibility.Visible,
    };
  
    popup.IsOpen = true;
    Debug.WriteLine( "GestureListener_Tap: " + text );
  }
  </code></pre>
  
  <p>The call to <code>WriteLine</code> prints in the debugger output window but the popup doesn't get displayed. What am I doing wrong here?</p>
  
  <p>Thanks for your help!</p>