XAML Element responsive width on Orientation change - xaml

Question

I am building a Windows 8.1 Universal app. On the orientation change for the phone, the outer "Grid" element doesn't change in width when the phone is switched to Landscape mode (and vice versa if the app is originally opened in landscape - doesn't change width when orientation=portrait). The Width is bound to the PivotItem Width and the PivotItem fills the entire screen on orientation change but the Grid does not.
<Page x:Name="page"
x:Class="App1.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:App1"
xmlns:m="using:App1.Models"
xmlns:vm="using:App1.ViewModels"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
DataContext="{Binding DefaultViewModel, RelativeSource={RelativeSource Self}}"
mc:Ignorable="d">
<Page.BottomAppBar>
<CommandBar Foreground="White" Background="{StaticResource CustomResource}">
<AppBarButton x:Name="BackButton" Label="Back" HorizontalAlignment="Center"
Click="BackButton_Click" Icon="Back" />
<AppBarButton x:Name="ForwardButton" Label="Forward" HorizontalAlignment="Center"
Click="ForwardButton_Click" Icon="Forward" />
<AppBarButton x:Name="HomeButton" Label="Home" HorizontalAlignment="Center"
Click="HomeButton_Click" Icon="Home" />
<AppBarButton x:Name="SettingsButton" Label="Settings" HorizontalAlignment="Center"
Click="SettingsButton_Click" Icon="Setting" />
</CommandBar>
</Page.BottomAppBar>
<Page.Resources>
<DataTemplate x:Key="myHeader">
<TextBlock Text="{Binding}" FontFamily="CustomFont" />
</DataTemplate>
<Style x:Key="PivotStyle" TargetType="Pivot">
<Setter Property="HeaderTemplate" Value="{StaticResource myHeader}"/>
</Style>
<!--<dict:NumberConverter x:Key="myNumberConverter" />-->
</Page.Resources>
<d:Page.DataContext>
<vm:DesignTimeDataSample />
</d:Page.DataContext>
<Grid>
<Pivot x:Name="MyPivot"
Style="{StaticResource PivotStyle}"
Background="{StaticResource CustomItem}"
SelectionChanged="MyPivot_SelectionChanged"
Foreground="Purple"
Grid.Row="1">
<PivotItem x:Name="Feed"
Header="news feed"
FontFamily="CustomFont">
<ScrollViewer Name="ScrollViewer">
<StackPanel x:Name="stackPanel">
<ListView ItemsSource="{Binding TileStories}" x:Name="cont" Margin="0,10,0,10" Background="{StaticResource CustomItem}" BorderBrush="{StaticResource CustomItem}" Foreground="{StaticResource CustomItem}">
<ListView.ItemContainerStyle>
<Style TargetType="ListViewItem">
<Setter Property="HorizontalAlignment" Value="Stretch" />
<Setter Property="HorizontalContentAlignment" Value="Stretch" />
<Setter Property="MinHeight" Value="0" />
<Setter Property="Margin" Value="0,10,0,10" />
<Setter Property="Foreground" Value="{StaticResource CustomItem}" />
</Style>
</ListView.ItemContainerStyle>
<ListView.ItemTemplate>
<DataTemplate>
<Border CornerRadius="20" BorderThickness="0" Background="White" HorizontalAlignment="Stretch">
<StackPanel Height="160" Orientation="Horizontal">
<Grid HorizontalAlignment="Stretch" Width="{Binding ActualWidth, ElementName=Feed, Mode=OneWay}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="2*" />
</Grid.ColumnDefinitions>
<Grid Grid.Column="0">
<Image Source="{Binding ImagePath}" Margin="20"/>
</Grid>
<Grid Grid.Column="1">
<Grid.RowDefinitions>
<RowDefinition Height="2*" />
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<Grid Grid.Row="0" Margin="0,0,0.333,53" Grid.RowSpan="2" HorizontalAlignment="Stretch">
<TextBlock VerticalAlignment="Center"
LineStackingStrategy="BlockLineHeight"
LineHeight="20" MaxHeight="80"
TextWrapping="WrapWholeWords"
Text="{Binding Headline}" FontSize="18"
FontFamily="CustomItem" />
</Grid>
<Grid Grid.Row="1" Margin="0,0.333,0.333,0">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="2*"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Grid Grid.Column="0">
<TextBlock VerticalAlignment="Center"
HorizontalAlignment="Left"
Margin="5" Text="{Binding Date}"
Foreground="#FFBDBDBD"
FontFamily="CustomItem" />
</Grid>
<Grid Grid.Column="1">
<AppBarButton
Style="{StaticResource CustomItem}"
VerticalAlignment="Center"
Margin="0,0,0,2"
HorizontalAlignment="Right"
Icon="Read"
CommandParameter="{Binding Url}"
Click="StoryButton_Click" FontFamily="Global User Interface"/>
</Grid>
</Grid>
</Grid>
</Grid>
</StackPanel>
</Border>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
<Button Foreground="White" Content="Send Notification" Grid.Row="0" HorizontalAlignment="Stretch" Name="ToastButton" Click="ToastButton_Click" Margin="0,0,0,25.333"/>
</StackPanel>
</ScrollViewer>
</PivotItem>
<PivotItem x:Name="News" Header="news">
<WebView x:Name="MyWebView" NavigationCompleted="MyWebView_NavigationCompleted"/>
</PivotItem>
</Pivot>
</Grid>
</Page>