Wrong UWP offset animation

For my application, I want to be able to use the Composition API to animate the displacement UIElement This element is predefined in Xaml and I found that the visual level of these controls is only calculated AFTER the animation has started ...

This causes the animation to only show if it was called a second time

My Xaml

<StackPanel x:Name="ActionButtonsPanel" Margin="50,175,0,0" HorizontalAlignment="Left" VerticalAlignment="Top">
    <Button x:Name="CreateNewButton" Tag="&#xE160;" Content="Create New..." Style="{StaticResource IconButtonStyle}" Click="CreateNewButton_Click"/>
    <Button x:Name="OpenFileButton" Tag="&#xE838;" Content="Open File..." Style="{StaticResource IconButtonStyle}" Margin="0,10,0,0" Click="OpenFileButton_Click"/>
</StackPanel>

      

        
        
        
      

    

My code

private void ShowNextButtons(UIElement Item1, UIElement Item2) {
    var _compositor = ElementCompositionPreview.GetElementVisual(this).Compositor;

    var visual1 = ElementCompositionPreview.GetElementVisual(Item1);
    visual1.CenterPoint = new Vector3(0, (float) Item1.RenderSize.Height / 2F, 0);

    var animationGroup1 = _compositor.CreateAnimationGroup();

    var offset1 = visual1.Offset; //First Time: Offset = <0,0,0>

    var fadeOut = _compositor.CreateScalarKeyFrameAnimation();
    fadeOut.Target = "Opacity";
    fadeOut.Duration = TimeSpan.FromMilliseconds(1000);
    fadeOut.InsertKeyFrame(0, 1);
    fadeOut.InsertKeyFrame(1, 0);
    //animationGroup1.Add(fadeOut); Not included: To be able to click a second time

    var slideOut = _compositor.CreateScalarKeyFrameAnimation();
    slideOut.Target = "Offset.X";
    slideOut.Duration = TimeSpan.FromMilliseconds(1000);
    slideOut.InsertKeyFrame(0, offset1.X);
    slideOut.InsertKeyFrame(1, offset1.X - 20F);
    animationGroup1.Add(slideOut);

    visual1.StartAnimationGroup(animationGroup1);
}

      

        
        
        
      

    

The animation Offset.X

is only displayed the second time the method was called