Resizing animation on SizeChanged event doesn't work as it should

Resizing animation on SizeChanged event doesn't work as it should

I want a resizing animation when stackpanel visibility is set to visible
state, but instead of that, I'm getting neverending flickering of border
containing stackpanel.I don't think I'm doing it wrong.
private void MyBorder_SizeChanged_1(object sender, SizeChangedEventArgs e)
{
if (!first)
{
DoubleAnimation anim = new DoubleAnimation();
anim.From = e.PreviousSize.Height;
anim.To = e.NewSize.Height;
anim.Duration = new Duration(TimeSpan.FromSeconds(1));
Storyboard.SetTarget(anim, MyBorder);
Storyboard.SetTargetProperty(anim, new
PropertyPath(Border.HeightProperty));
Storyboard st = new Storyboard();
st.Children.Add(anim);
st.Begin();
}
first = false;
}
private void MyBorder_Tap_1(object sender, GestureEventArgs e)
{
if (MyPanel.Visibility == Visibility.Collapsed)
MyPanel.Visibility = Visibility.Visible;
else
MyPanel.Visibility = Visibility.Collapsed;
}