title: scale animation for wpf popup
body: <p>I have a nice little popup, when it shows, I d'like it to growth from 0 to 1x scaley,
  but I don't get it right, when I click multiple times, it looks like i "catch" the animation at
  various states during the "growth".</p>
  
  <pre><code>&lt;Window.Triggers&gt;
  
      &lt;EventTrigger RoutedEvent="FrameworkElement.MouseRightButtonDown" 
                    &gt;
          &lt;EventTrigger.Actions&gt;
              &lt;BeginStoryboard&gt;
                  &lt;Storyboard&gt;
                      &lt;DoubleAnimationUsingKeyFrames BeginTime="00:00:00" 
                          Storyboard.TargetName="SimplePopup" 
                          Storyboard.TargetProperty="(FrameworkElement.LayoutTransform).(TransformGroup.Children)[0].(ScaleTransform.ScaleY)"&gt;
                          &lt;SplineDoubleKeyFrame KeyTime="00:00:00" Value="0"/&gt;
                          &lt;SplineDoubleKeyFrame KeyTime="00:00:00.3000000" Value="1"/&gt;
                      &lt;/DoubleAnimationUsingKeyFrames&gt;
  
                  &lt;/Storyboard&gt;
              &lt;/BeginStoryboard&gt;
          &lt;/EventTrigger.Actions&gt;
      &lt;/EventTrigger&gt;
  
  &lt;/Window.Triggers&gt;
  </code></pre>
  
  <p>and the popup:</p>
  
  <pre><code>   &lt;Popup Name="SimplePopup"
             AllowsTransparency="True" 
             StaysOpen="False"&gt;
          &lt;Popup.LayoutTransform&gt;
              &lt;TransformGroup&gt;
                  &lt;ScaleTransform ScaleX="1" ScaleY="1" /&gt;
                  &lt;SkewTransform AngleX="0" AngleY="0" /&gt;
                  &lt;RotateTransform Angle="0" /&gt;
                  &lt;TranslateTransform X="0" Y="0" /&gt;
              &lt;/TransformGroup&gt;
          &lt;/Popup.LayoutTransform&gt;
          &lt;Border&gt; some Content here
            &lt;/Border&gt;
      &lt;/Popup&gt;
  </code></pre>