A thumb where the look can depend on the IsPrimarySelection property.
Inheritance: System.Windows.Controls.Primitives.Thumb
		protected DesignerThumb CreateThumb(PlacementAlignment alignment, Cursor cursor)
		{
			DesignerThumb designerThumb = new DesignerThumb { Alignment = alignment, Cursor = cursor, IsPrimarySelection = true};
			AdornerPanel.SetPlacement(designerThumb, Place(designerThumb, alignment));

			adornerPanel.Children.Add(designerThumb);

			DragListener = new DragListener(designerThumb);
			DragListener.Started += drag_Started;
			DragListener.Changed += drag_Changed;
			DragListener.Completed += drag_Completed;
			
			return designerThumb;
		}
		/// <summary>
		/// Places resize thumbs at their respective positions
		/// and streches out thumbs which are at the center of outline to extend resizability across the whole outline
		/// </summary>
		/// <param name="designerThumb"></param>
		/// <param name="alignment"></param>
		/// <param name="index">if using a polygon or multipoint adorner this is the index of the point in the Points array</param>
		/// <returns></returns>
		protected PointTrackerPlacementSupport Place(ref DesignerThumb designerThumb, PlacementAlignment alignment, int index = -1)
		{
			PointTrackerPlacementSupport placement = new PointTrackerPlacementSupport(ExtendedItem.View as Shape, alignment, index);
			return placement;
		}
		/// <summary>
		/// Places resize thumbs at their respective positions
		/// and streches out thumbs which are at the center of outline to extend resizability across the whole outline
		/// </summary>
		/// <param name="designerThumb"></param>
		/// <param name="alignment"></param>
		/// <returns></returns>
		private RelativePlacement Place(ref DesignerThumb designerThumb,PlacementAlignment alignment)
		{
			RelativePlacement placement = new RelativePlacement(alignment.Horizontal,alignment.Vertical);
			
			if (alignment.Horizontal == HorizontalAlignment.Center)
			{
				placement.WidthRelativeToContentWidth = 1;
				placement.HeightOffset = 6;
				designerThumb.Opacity = 0;
				return placement;
			}
			if (alignment.Vertical == VerticalAlignment.Center)
			{
				placement.HeightRelativeToContentHeight = 1;
				placement.WidthOffset = 6;
				designerThumb.Opacity = 0;
				return placement;
			}
			
			placement.WidthOffset = 6;
			placement.HeightOffset = 6;
			return placement;
		}
		/// <summary>
		/// is invoked whenever a line is selected on the canvas, remember that the adorners are created for each line object and never destroyed
		/// </summary>
		protected override void OnInitialized()
		{
			base.OnInitialized();
			
			resizeThumbs = new DesignerThumb[]
			{
				CreateThumb(PlacementAlignment.TopLeft, Cursors.Cross),
				CreateThumb(PlacementAlignment.BottomRight, Cursors.Cross)
			};

			extendedItemArray[0] = this.ExtendedItem;

			Invalidate();
			
			this.ExtendedItem.PropertyChanged += OnPropertyChanged;
			resizeBehavior = PlacementOperation.GetPlacementBehavior(extendedItemArray);
			UpdateAdornerVisibility();
		}