You must extend the design mode behavior by using ParentControlDesigner
Class. ParentControlDesigner Class provides a base class for designers of controls that can contain child controls.
So, for achieve to your Goal, you must implement design-time services for a component by DesignerAttribute
as the following (Just add below code before the written class):
[Designer("System.Windows.Forms.Design.ParentControlDesigner, System.Design", typeof(IDesigner))]
public partial class barFloorsGrouping : Bar