Hi,

The problem in your case is that a selected item is not being modified. The cache image used for fast drawing during interaction contains all items not being modified, and they are drawn without selection handles since the control expects they will be rendered for the modified items later. One way to circumvent this is to override the diagram's Draw method, which is called when creating the cache image:

namespace TestCustomBehaviour
{
	class MyDiagram: Diagram
	{
		public override void Draw(IGraphics graphics, RenderOptions options, RectangleF clipRect, bool noModifiedItems)
		{
			base.Draw(graphics, options, clipRect, noModifiedItems);

			if (noModifiedItems && ActiveItem != null && !ActiveItem.NowModifying())
			{
				// draw the handles of ActiveItem in modification cache image
				ActiveItem.DrawHandles(graphics, ActiveItemHandlesStyle);
			}
		}
	}
} 

and also remove the this.diagram1.SelectionOnTop = false line, which prevents correct order of selection handles drawn in the cache.

I hope that helps,
Stoyan