gistfile1.txt

<ItemsControl ItemsSource="{Binding Rows}"
      Grid.Column="1"
          HorizontalAlignment="Center">

...

<Popup HorizontalAlignment="Center" 
   VerticalAlignment="Center" 
   VerticalOffset="-150" 
   HorizontalOffset="-100" 
   IsOpen="{Binding IsSolved}" 
   Grid.Column="1">

...

<StackPanel Orientation="Horizontal">
    <Button Style="{StaticResource UndoAppBarButtonStyle}" Command="{Binding Undo}" />
    
...