I created a simple white control with a black border derived from HwndHost (named LightGrid) and added it to ScrollViewer along with some other controls (replaced with “…”) as shown below:
<ScrollViewer HorizontalScrollBarVisibility="Disabled">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
...
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
...
</Grid.RowDefinitions>
...
<Border Grid.Row="6" Grid.ColumnSpan="2" Visibility="Visible" Height="200">
<nc:LightGrid />
</Border>
</Grid>
</ScrollViewer>
It looks correctly in the lower scroll position, and it even can scroll inside ScrollViewer (it moves when I move the scroll bar), but when scrolled, it is not clipped so it obscures other controls:
