StackPanel Orientation stops page load - xaml

Question

First to explain what I am doing here, I want to create a page that list a large number of grouped objects. Selecting an object binds that information to a display on the left. Everything works fine till I change the MainStack Orientation to Horizontal. Once that is changed the page no longer loads. No errors are thrown. When stepping through the process the code behind steps through as it should.
I know its the XAML but am baffled by the cause. It works fine under two circumstances.
If I drop the mainstack stack panel and instead make the display group the header of the GridView it works.
If I put the MainStack orientation to Vertical it loads fine.
Here is the code that does not load:
<StackPanel Name="MainStack" Orientation="Horizontal" Grid.Row="2" >
<StackPanel Name="Stack" Width="480" >
<TextBlock Text="{Binding Nname}" Margin="0,0,0,20" Style="{StaticResource SubheaderTextBlockStyle}" MaxHeight="60"/>
<TextBlock Text="{Binding Nset}" Margin="0,0,0,20" Style="{StaticResource SubheaderTextBlockStyle}" MaxHeight="60"/>
<Image Source="{Binding url}" Height="Auto" Margin="0,0,102,55" Stretch="UniformToFill" AutomationProperties.Name="{Binding Title}"/>
<ScrollViewer Margin="0,0,0,0" MaxHeight="200">
<TextBlock Text="{Binding Nruling}" Margin="0,0,0,0" Style="{StaticResource BodyTextBlockStyle}"/>
</ScrollViewer>
</StackPanel>
<SemanticZoom x:Name="semanticZoom" Width="Auto" >
<SemanticZoom.ZoomedOutView>
<GridView Foreground="White"
ScrollViewer.IsHorizontalScrollChainingEnabled="False">
<GridView.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Group.Key}"
FontFamily="Segoe UI" FontWeight="Light" FontSize="24" />
</DataTemplate>
</GridView.ItemTemplate>
<GridView.ItemsPanel>
<ItemsPanelTemplate>
<ItemsWrapGrid ItemWidth="100" ItemHeight="100" MaximumRowsOrColumns="4"/>
</ItemsPanelTemplate>
</GridView.ItemsPanel>
<GridView.ItemContainerStyle>
<Style TargetType="GridViewItem">
<Setter Property="Margin" Value="4" />
<Setter Property="Padding" Value="10" />
<Setter Property="Background" Value="#FF25A1DB" />
<Setter Property="BorderThickness" Value="1" />
<Setter Property="HorizontalContentAlignment" Value="Left" />
<Setter Property="VerticalContentAlignment" Value="Bottom" />
</Style>
</GridView.ItemContainerStyle>
</GridView>
</SemanticZoom.ZoomedOutView>
<SemanticZoom.ZoomedInView>
<GridView
x:Name="itemGridView"
AutomationProperties.AutomationId="ItemGridView"
AutomationProperties.Name="Items In Group"
TabIndex="1"
Padding="120,126,120,50"
ItemsSource="{Binding Source={StaticResource itemsViewSource}}"
SelectionMode="Single"
ScrollViewer.IsHorizontalScrollChainingEnabled="False" SelectionChanged="itemGridView_SelectionChanged">
<GridView.ItemsPanel>
<ItemsPanelTemplate>
<ItemsWrapGrid MaximumRowsOrColumns="8" GroupHeaderPlacement="Top" />
</ItemsPanelTemplate>
</GridView.ItemsPanel>
<GridView.ItemTemplate>
<DataTemplate>
<Grid Height="110" Width="480" Margin="10">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Border Background="{ThemeResource ListViewItemPlaceholderBackgroundThemeBrush}" Width="110" Height="110">
<Image Source="{Binding url}" Stretch="UniformToFill" AutomationProperties.Name="{Binding Nname}"/>
</Border>
<StackPanel Grid.Column="1" VerticalAlignment="Top" Margin="10,0,0,0">
<TextBlock Text="{Binding Nname}" Style="{StaticResource TitleTextBlockStyle}" TextWrapping="NoWrap"/>
<TextBlock Text="{Binding Nset}" Style="{StaticResource CaptionTextBlockStyle}" TextWrapping="NoWrap"/>
<TextBlock Text="{Binding Nruling}" Style="{StaticResource BodyTextBlockStyle}" MaxHeight="60"/>
</StackPanel>
</Grid>
</DataTemplate>
</GridView.ItemTemplate>
<GridView.ItemContainerStyle>
<Style TargetType="FrameworkElement">
<Setter Property="Margin" Value="52,0,0,2"/>
</Style>
</GridView.ItemContainerStyle>
<GridView.GroupStyle>
<GroupStyle>
<GroupStyle.HeaderTemplate>
<DataTemplate>
<Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}" Margin="10">
<TextBlock Text='{Binding Key}' Foreground="{StaticResource ApplicationForegroundThemeBrush}" FontSize="25" Margin="5" />
</Grid>
</DataTemplate>
</GroupStyle.HeaderTemplate>
</GroupStyle>
</GridView.GroupStyle>
</GridView>
</SemanticZoom.ZoomedInView>
</SemanticZoom>
<!-- Horizontal scrolling grid -->
</StackPanel>