The easiest thing to do is to greatly increase the value of Diagram.Padding. Maybe something like Padding=“1000”.
The Diagram.Padding is used by the Diagram.Panel in its DiagramPanel.ComputeDiagramBounds() method. That is what normally determines the value of DiagramPanel.DiagramBounds, which is the value used by the scrollbars to limit where the user can scroll/pan. By making the DiagramBounds bigger than needed, users will be able to scroll the diagram “contents” further within the viewport.
If this does not produce the results that you want, we can be more sophisticated about this.