C#WPF里不能出现滚动条的原因

使用下面这段代码，就不能出现滚动条：

            <mdix:DrawerHost.LeftDrawerContent><Grid Width="260" Background="{StaticResource MaterialDesign.Brush.Primary}"><Grid.RowDefinitions><RowDefinition Height="auto"/></Grid.RowDefinitions><ScrollViewer Grid.Row="0" HorizontalAlignment="Stretch" HorizontalScrollBarVisibility="Disabled" VerticalScrollBarVisibility="Auto" ><StackPanel x:Name="Menu" Margin="10"/></ScrollViewer></Grid>