AnimatedExpander keeps Width after being collapsed

Hello,

I am using the AnimatedExpander with a content that is wider than the containing scroll viewer. So a horizontal scroll bar appears when I fill the content on expanding. When I collapse the AnimatedExpander the scroll bar doesn't change, as if the content was still visible. I believe that the AnimatedExpander retains its width even when collapsed.

I can evade this problem by binding the Visibility of the content to the IsExpanded of the AnimatedExpander.

Still I wonder if this is an issue of the AnimatedExpander or is there maybe a way to force the AnimatedExpander to use its Header for Width when it is not expanded?

<ScrollViewer HorizontalAlignment="Stretch" VerticalAlignment="Stretch" CanContentScroll="False" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
<StackPanel>
<shared:AnimatedExpander Header="myHeader">
<Grid ...

Any hints are most welcome.