如何使 WPF Expander 在保持标题固定的同时向上扩展

<Expander Header="标题">
    <!-- 内容 -->
</Expander>

<Expander Header="标题">
    <Expander.Template>
        <ControlTemplate TargetType="Expander">
            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition Height="Auto"/>
                    <RowDefinition Height="Auto"/>
                </Grid.RowDefinitions>
                <ToggleButton x:Name="ExpanderButton" Content="{TemplateBinding Header}" IsChecked="{Binding IsExpanded, RelativeSource={RelativeSource TemplatedParent}}" />
                <ContentPresenter x:Name="ContentPresenter" Grid.Row="1" VerticalAlignment="Top" Margin="0,-20,0,0"/>
            </Grid>
        </ControlTemplate>
    </Expander.Template>
    <!-- 内容 -->
</Expander>