title: WPF Control disappears when is is animated with DoubleAnimation
body: <p>I have a TextBox(toAnimate) and an Button(toClick).
  When i click the button, the TextBox disappears.
  But it should move on the X Position 20 units.<p>
  What did i do wrong?</p>
  
  <pre><code>//double left = Canvas.GetLeft(toAnimate); Returned wrong value.
  TranslateTransform tanl = new TranslateTransform();
  Vector pos = VisualTreeHelper.GetOffset(toAnimate);
  double left = pos.X;
  toAnimate.RenderTransform = tanl;
  DoubleAnimation doua = new DoubleAnimation((left + 20), left, TimeSpan.FromSeconds(3));
  tanl.BeginAnimation(TranslateTransform.XProperty, doua);
  </code></pre>