Code

class RectanglesBehavior : DrawShapesBehavior
{
	public RectanglesBehavior(Diagram diagram) : base(diagram)
	{
	}

	public override InteractionState StartDraw(Point point)
	{
		Diagram.ModifierKeyActions.Control = ModifierKeyAction.None;
		if (Keyboard.Modifiers == ModifierKeys.Control)
			return new InteractionState(new ShapeNode(Diagram), null, Action.Create);

		var selected = Diagram.ActiveItem as ShapeNode;
		if (selected != null )
		{
			var handle = selected.HitTestHandle(point);
			if (handle != null)
				return new InteractionState(selected, handle, Action.Modify);
		}

		/*
		var rect = NearestRect(point);
		if (rect != null)
		{
			var handlesStyle = rect.HandlesStyle;
			rect.HandlesStyle = HandlesStyle.SquareHandles2;
			var handle = rect.HitTestHandle(point);
			rect.HandlesStyle = handlesStyle;
			if (handle != null)
				return new InteractionState(rect, handle, Action.Modify);
		}*/

		return base.StartDraw(point);
	}

	/*public override CursorHint SetMouseCursor(Point point, out bool startInteraction)
	{
		return base.SetMouseCursor(point, out startInteraction);
	}*/

	/*protected override void OnMouseUp(Point mousePosition, MindFusion.Diagramming.Wpf.MouseButton mouseButton)
	{
		base.OnMouseUp(mousePosition, mouseButton);
	}*/

	DiagramNode NearestRect(Point point)
	{
		DiagramNode nearest = null;
		double minDist = double.MaxValue;
		foreach (var node in Diagram.Nodes)
		{
			if (node.Locked)
				continue;
			var dist = DistToRect(point, node.Bounds);
			if (dist < minDist)
			{
				minDist = dist;
				nearest = node;
			}
		}
		return nearest;
	}

	double DistToRect(Point point, Rect rect)
	{
		var v = new[]
        {
        	new Point(rect.Left, rect.Top),
			new Point(rect.Right, rect.Top),
			new Point(rect.Right, rect.Bottom),
			new Point(rect.Left, rect.Bottom),
        };
		double minDist = double.MaxValue;
		for (var i = 0; i < 4; i++)
		{
			var v1 = v[i];
			var v2 = v[(i + 1) % 4];
			var dist = MindFusion.Utilities.DistToLineSegment(point, v1, v2);
			if (dist < minDist)
				minDist = dist;
		}
		return minDist;
	}
}

diagram.ModifierKeyActions.Control = ModifierKeyAction.None;
diagram.CustomBehavior = new RectanglesBehavior(diagram);