You cannot set height properties when working with ScrollViewer. If you divide it into a few below, and it still does not work, then this is not all that interferes with your project.
<ScrollViewer>
<Grid>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<TextBlock Text="Hello" FontSize="320" />
<TextBlock Grid.Row="1" Text="World" FontSize="320" />
</Grid>
</ScrollViewer>
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