VisualBrush of an expander..

Problem:

When I create a visualbrush of (an initially) collapsed Expander (with no
explicit Height set) The "first" render of the visualbrush will render the
header only. After I expanded and collapsed the expander back. The
visualbrush renders the header AND the whitespace the Expander control took
when it was expanded. It "remembers" the height someway. Is there a way that
the visualbrush only renders the header again?

I prepared a little test xaml to show the problem. Just expand and collapse
the Expander. The rectangle will be different from the original one.



<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xml:lang="en-US"
x:Class="testvisualbrush.Window1"
x:Name="Window"
Title="Window"
Width="400" Height="200">


<Canvas>
<Expander Canvas.Top="0" Width="300" Name="exp" Background="#FFFF8B00">
<Expander.Header>Header</Expander.Header>
<Label>Content
</Label>
</Expander>
<Rectangle Canvas.Top="50" Stroke="#FF000000" Width="300" Height="100">

<Rectangle.Fill>
<VisualBrush><VisualBrush.Visual>
<Binding ElementName="exp" />
</VisualBrush.Visual></VisualBrush>
</Rectangle.Fill>
</Rectangle>
</Canvas>
</Window>