Hello Wu,

Thank you for the provided information. The diagram doesn't have a document area defined where you can interact with elements. The control basically provides an infinity surface where you can place shapes and connections.

You can mimic a behavior similar to the Visio one by creating a custom shape that will cover the document area and restrict shapes only in this area. You can see how to restrict the shape's dragging in the CustomServices SDK example (https://github.com/telerik/xaml-sdk/tree/master/Diagram/CustomServices). If you have struggles with this approach I would ask you to open a new support ticket and provide additional information and maybe few images on your scenario. 

Also, you can check the RadImageEditor control (https://docs.telerik.com/devtools/wpf/controls/radimageeditor/overview) and see if this is going to be suitable for your case.

I am declining this item because it sounds more like a customization question then a feature request. Please correct me if I am wrong.

Regards,
Martin Ivanov
Technical Support Engineer