Hi,

That area is not part of the control and the Diagram object does not receive mouse events raised there, so the only way to allow selection is to set larger Bounds or call diagram.ResizeToFitItems. You could also set ClipToBounds to prevent items outside of the diagram area from being shown. If you are specifically looking for an effect where a part of the diagram background is drawn in one color and another part in a different color, you can handle the DrawBackground event to achieve that.

I hope that helps,
Stoyan