WPF Scrollviewer not working with dynamic height - wpf

Question

Trying to add a Scrollviewer to a TextBlock so that users can scroll down the contents, which are often rather longer than the available screen real-estate.
Apologies for what's probably a dumb question: I can see there's lots of topics about this, and that the problem is usually a fixed height somewhere, but I'm struggling to see which element is causing the problem in my XAML:
<Popup StaysOpen="True" Placement="Center" IsOpen="{Binding SummaryOpen}" PlacementTarget="{Binding ElementName=Areas}">
<Border Background="LightGray" BorderBrush="Black" Padding="5" BorderThickness="1">
<Grid Width="500">
<Grid.RowDefinitions>
<RowDefinition Height="50" />
<RowDefinition Height="350" />
<RowDefinition Height="40" />
</Grid.RowDefinitions>
<StackPanel Grid.Row="0" Orientation="Horizontal">
<Label Content="{Binding Name}" />
<Label Content=": " />
<Label Content="{Binding Description}" />
</StackPanel>
<Border Grid.Row="1" BorderBrush="Black" BorderThickness="1">
<StackPanel Background="White" Margin="-1,1,1,-1">
<!-- this is the rogue element -->
<ScrollViewer VerticalScrollBarVisibility="Auto">
<TextBlock Text="{Binding Summary}" TextWrapping="Wrap" />
</ScrollViewer>
</StackPanel>
</Border>
</Grid>
</Border>
</Popup>
The ScrollViewer appears, but never contains an actual scroll bar, regardless of how much content there is in the TextBlock.
If someone could explain where the problem is and how you fix it, I'd be most appreciative.

Answer 1

The rogue element is actually the parent StackPanel -- that panel isn't "fixed height" per se, but it doesn't work as a parent of a ScrollViewer. The reason is that it reports its available height as infinite, so the child ScrollViewer thinks it can extend as far as its children require, and so it doesn't need to scroll.
It looks like you could just as easily use a Border, or a Grid, either of which will limit their height to the parent height and thus fix the issue:
<Border Grid.Row="1" BorderBrush="Black" BorderThickness="1">
<Border Background="White" Margin="-1,1,1,-1">
<!-- this is the rogue element -->
<ScrollViewer VerticalScrollBarVisibility="Auto">
<TextBlock Text="{Binding Summary}" TextWrapping="Wrap" />
</ScrollViewer>
</Border>
</Border>