XML grid won't expand - winrt-xaml

Question

I have a parent grid ("Output") hosting 2 smaller grids.
The smaller grids MainDisplay (which basically shows an image in a ViewBox that I want to take all of the horizontal space except for 300 in the other of the 2 sub grids) and Input (a fixed width of 300, which actually works as expected).
My problems are:
1) The parent grid does not consume all the horizontal space. I want the extra space to go to the grid with the ViewBox.
I have sprinkled 'HorizontalAlignment="Stretch"' and 'Width="*"' all over, but it won't take up all the space on the screen!
2) I have code behind to grab the width/height of the image. Unless I explicitly set the height, width the code behind tries to grab the height and width and crashes ... but if I set the width/height explicitly I'm afraid it's not going to grab the available space.
Here is the code:
<Grid x:Name="Output" Background="#1D1D1D" Margin="0,2,0,-2" HorizontalAlignment="Stretch">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="300"/>
</Grid.ColumnDefinitions>
<Grid x:Name="MainDisplay" HorizontalAlignment="Stretch" VerticalAlignment="Top" Grid.Column="0">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Viewbox Width="Auto" Height="Auto" MinWidth="400" MinHeight="400" x:Name="Scenario4ImageContainer" Stretch="Uniform">
<Image x:Name="Scenario4Image" PointerPressed="Scenario4Image_PointerPressed" HorizontalAlignment="Stretch" />
</Viewbox>
</Grid>
<Grid Width="300" x:Name="Input" Grid.Column="1">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<StackPanel Grid.Row="0">
<TextBlock Style="{StaticResource BasicTextStyle}" TextWrapping="Wrap"
Text="Tools" />
<Grid Margin="0,10,0,0">
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="230" />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Button Grid.Row="0" Grid.Column="0" HorizontalAlignment="Stretch" Background="DarkSlateGray"
Content="Draw Mandelbrot set" Click="DrawMandelbrotSet_Click" />
<TextBlock x:Name="Scenario4DrawMandelbrotDescription" Grid.Row="0" Grid.Column="1"
Style="{StaticResource BasicTextStyle}" TextWrapping="Wrap" VerticalAlignment="Center"
Text="Text." />
<Button x:Name="Scenario4SetSourceButton" Grid.Row="1" Grid.Column="0" HorizontalAlignment="Stretch"
Content="Load image using SetSource" Click="LoadImageUsingSetSource_Click" />
<TextBlock x:Name="Scenario4SetSourceDescription" Grid.Row="1" Grid.Column="1"
Style="{StaticResource BasicTextStyle}" TextWrapping="Wrap" VerticalAlignment="Center"
Text="Text." />
<Button x:Name="Scenario4LoadImageButton" Grid.Row="2" Grid.Column="0" HorizontalAlignment="Stretch"
Content="Load image using PixelBuffer" Click="LoadImageUsingPixelBuffer_Click" />
<TextBlock x:Name="Scenario4LoadImageDescription" Grid.Row="2" Grid.Column="1"
Style="{StaticResource BasicTextStyle}" TextWrapping="Wrap" VerticalAlignment="Center"
Text="Text." />
</Grid>
</StackPanel>
</Grid>

Answer 1

I'm not 100% sure on this, but try "*" instead of "Auto" in your first column definition:
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="300"/>
</Grid.ColumnDefinitions>