<Menu.Resources>
    <Style TargetType="MenuItem">
        <Setter Property="HorizontalAlignment" Value="Stretch"/>
        <Setter Property="Background" Value="Blue"/>
        <Setter Property="HeaderTemplate">
            <Setter.Value>
                <DataTemplate>
                    <Border>
                        <TextBlock Text="{Binding}" HorizontalAlignment="Center"/>
                    </Border>
                </DataTemplate>
            </Setter.Value>
        </Setter>
    </Style>

public class MenuPopupPlacementAttachedProperty : BaseAttachedProperty<MenuPopupPlacementAttachedProperty, PlacementMode>
{
	private Menu menu = null;

	public override void OnValueChanged( DependencyObject d, DependencyPropertyChangedEventArgs e )
	{
		menu = d as Menu;

		if( menu.IsLoaded )
		{
			SubscribeChildren();
		}
		else
		{
			RoutedEventHandler handler = null;
			handler = (oo, ee) =>
			{
				SubscribeChildren();
				menu.Loaded -= handler;
			};
			menu.Loaded += handler;
		}
	}

    class GridEx : Grid
    {
        public static string GetLocation(DependencyObject obj) => (string)obj.GetValue(LocationProperty);
        public static void SetLocation(DependencyObject obj, string value) => obj.SetValue(LocationProperty, value);

        public static readonly DependencyProperty LocationProperty =
            DependencyProperty.RegisterAttached(
                "Location", typeof(string), typeof(GridEx),
                new PropertyMetadata(string.Empty, (o, args) => EnsureParentSubscription((FrameworkElement)o)));

<local:GridEx Background="Gray">
    <local:GridEx.RowDefinitions>
        <RowDefinition/>
        <RowDefinition/>
        <RowDefinition/>
    </local:GridEx.RowDefinitions>
    <local:GridEx.ColumnDefinitions>
        <ColumnDefinition/>
        <ColumnDefinition/>
        <ColumnDefinition/>
    </local:GridEx.ColumnDefinitions>
    <Rectangle Fill="Red"/>
    <Rectangle Fill="Green" local:GridEx.Location="next"/>
    <TextBlock Text="Working!" local:GridEx.Location="current"/>
    <Rectangle Fill="Yellow" local:GridEx.Location="1,1"/>

if (location == string.Empty)
{
    currColumn = Grid.GetColumn(child);
    currRow = Grid.GetRow(child);
    continue;
}