Scrollviewer hides my following controls

See more:

C#

XAML

WPF

DockPanel

Scrollbar

Hello!

So here is my code:

<DockPanel>
        <ScrollViewer DockPanel.Dock="Top" VerticalScrollBarVisibility="Auto">
            <StackPanel>
                <ListBox Name="MixControl" ScrollViewer.HorizontalScrollBarVisibility="Disabled" HorizontalAlignment="Stretch" ItemContainerStyle="{StaticResource StretchedContainerStyle}" ItemTemplate="{StaticResource MixListBoxItemTemplate}"/>
            </StackPanel>
        </ScrollViewer>
        <Expander DockPanel.Dock="Bottom" VerticalAlignment="Bottom" IsExpanded="True">
            <DockPanel>
                <TextBlock Text="Login:"></TextBlock>
                <TextBox></TextBox>
                <TextBlock Text="Password:"></TextBlock>
                <PasswordBox></PasswordBox>
            </DockPanel>
        </Expander>
    </DockPanel>

I want to achieve to have a "band" on the bottom of my Window always visable. Above should be the Scrollviewer, showing the scrollbar when nessecary.

But if the scrollbar is shown, my bottom Dockpanel gets "hidden" by the Scrollviewer. What can I do to achieve always visability for my Expander?

Thanks!