Code

diagram.CustomBehavior = new MyModifyBehavior(diagram);

class MyModifyBehavior : ModifyBehavior
{
	protected internal MyModifyBehavior(Diagram diagram) :
		base(diagram)
	{
	}

	protected override void OnMouseMove(Point mousePosition)
	{
		base.OnMouseMove(mousePosition);

		if (Diagram.Interaction != null &&
			Diagram.Interaction.CurrentItem is Selection &&
			Diagram.Interaction.Action == Action.Create)
		{
			if (!Diagram.Bounds.Contains(Diagram.Selection.Bounds))
			{
				var r = Diagram.Bounds;
				r.Union(Diagram.Selection.Bounds);
				Diagram.Bounds = r;
			}
		}
	}
}