为什么＆＃34;风景＆＃34; position不调整LongListSelector的大小？

Question

为什么当我将手机置于横向位置时，LongListSelectornão会调整大小？

以下图片：

关注XAML：

<Grid x:Name="ContentPanel" Grid.Row="1" Margin="0" HorizontalAlignment="Center" VerticalAlignment="Center">
    <phone:LongListSelector Name="lstConsPais"
    HorizontalAlignment="Left" 
    VerticalAlignment="Top" 
    LayoutMode="List" 
    IsGroupingEnabled="False"
    Width="456" SelectionChanged="lstConsPais_SelectionChanged">
                <phone:LongListSelector.ItemTemplate>
                    <DataTemplate>
                        <Border BorderBrush="#111" Margin="0, 10, 0, 0" BorderThickness="0,0,0,2">
                            <StackPanel VerticalAlignment="Center" Orientation="Horizontal">
                                <Image Height="100" Width="100"  Source="{Binding NomeImgBandeira}" VerticalAlignment="Center" HorizontalAlignment="Left"/>
                                <StackPanel Orientation="Vertical">
                                    <TextBlock Text="{Binding NomePais}" Style="{StaticResource PhoneTextGroupHeaderStyle}" HorizontalAlignment="Left" VerticalAlignment="Center" Margin="15,25,0,0" />
                                </StackPanel>
                            </StackPanel>
                        </Border>

                    </DataTemplate>
                </phone:LongListSelector.ItemTemplate>
            </phone:LongListSelector>
        </Grid>

我希望他填满整个屏幕。

或多或少是这样的：

但我不想篡改“宽度”，我想“自动”