Expand stackpanel and child controls to fill screen width on various devices

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Windows

WinMobile

Mobile

XAML

Working on a windows universal app. I'm having difficulty in expanding a stackpanel to occupy the entire screen width on various devices. I'd like for the contents of the stackpanel to fill the entire screen for all devices.

XAML:

<Grid x:Name="RootLayout" Background="LightGray">
      <Grid.RowDefinitions>
          <RowDefinition Height="70" />
          <RowDefinition Height="55" />
          <RowDefinition Height="*" />
      </Grid.RowDefinitions>
      <Grid.ColumnDefinitions>
          <ColumnDefinition Width="*" />
      </Grid.ColumnDefinitions>
      <!-- Title Bar -->
      <Grid x:Name="TitleBar" Grid.Row ="0" Background="#FF2D8F9A">
          <TextBlock x:Name="txtTitle" TextWrapping="Wrap" Text="News" SelectionHighlightColor="{x:Null}" HorizontalAlignment="Center" VerticalAlignment="Center" FontSize="24"/>
      </Grid>
      <Grid x:Name="Toogle" Grid.Row ="1" Background="#FFCCCCCC" HorizontalAlignment="Center" VerticalAlignment="Center" Width="400">
          <ToggleButton x:Name="tbNews" Margin="202,-2,34,-1" Background="#FFB9B6B6" BorderBrush="Black" Content="In The News" Height="58" IsDoubleTapEnabled="False" IsHoldingEnabled="False" IsRightTapEnabled="False" Click="tbNews_Click" Style="{StaticResource CHSToggleButtonStyle}" HorizontalAlignment="Stretch"/>
          <ToggleButton x:Name="tbAnnouncements" Margin="0,-2,197,-1" Background="LightGray" BorderBrush="Black" Content="Announcements" Width="164" Height="58" FontFamily="Global User Interface" IsDoubleTapEnabled="False" IsHoldingEnabled="False" IsRightTapEnabled="False" IsChecked="False" Click="tbAnnouncements_Tapped" Style="{StaticResource CHSToggleButtonStyle}" HorizontalAlignment="Right"/>
      </Grid>
      <Grid x:Name="Content" Grid.Row ="2" Margin="0,0,0,0" HorizontalAlignment="Center">
          <StackPanel Name="Items" Orientation="Horizontal" HorizontalAlignment="Stretch">
              <ListBox HorizontalAlignment="Stretch" Name="Articlelist" BorderThickness="1" IsDoubleTapEnabled="False" IsHoldingEnabled="False" IsRightTapEnabled="False" FontFamily="Global User Interface" Tapped="Phonelist_Tapped" HorizontalContentAlignment="Stretch" VerticalContentAlignment="Top" Margin="0,0,0,0"  >
                  <ListBox.ItemTemplate>
                      <DataTemplate>
                          <Border BorderThickness="0, 0, 0, 1" BorderBrush="Black">
                              <StackPanel Orientation="Horizontal">
                                  <StackPanel HorizontalAlignment="Stretch" Name="LocationDetails" VerticalAlignment="Center" Margin="0">
                                      <TextBlock Height="auto" Width="350"  Text="{Binding Title}" VerticalAlignment="Center" HorizontalAlignment="Left" TextWrapping="WrapWholeWords" Foreground="Black"  FontSize="18" FontFamily="Global User Interface" SelectionHighlightColor="#FFD7D6D8" Margin="6" FontWeight="Bold" Padding="0,0,0,0" />
                                      <TextBlock Height="auto" Width="350" Foreground="Black" FontSize="18" Text="{Binding Teaser}" FontFamily="Global User Interface" Margin="6" SelectionHighlightColor="#FFF8F7F9" TextWrapping="WrapWholeWords" Padding="2" HorizontalAlignment="Left"/>
                                  </StackPanel>
                                  <Image Source="Assets/rt_carrot~ipad.png" Height="25" Width="15" VerticalAlignment="Center" Margin="6" Stretch="UniformToFill" HorizontalAlignment="Center"/>
                              </StackPanel>
                          </Border>
                      </DataTemplate>
                  </ListBox.ItemTemplate>
              </ListBox>
          </StackPanel>
      </Grid>
  </Grid>