I set Diagram.WheelBehavior=zoom.
This is due to the zoom commands depending on the DiagramPanel.Scale setter trying to limit any new scale value to between DiagramPanel.MinimumScale and MaximumScale. The Scale value actually got set to value just beyond the permitted range, causing an adjustment to the DiagramPanel.Position, before the Scale value was reset to its old value.
This is because Silverlight does not have a way of preventing a dependency property from being set to an invalid value.
For version 1.2 we’ll change the implementation of the zoom commands not to even try setting the DiagramPanel.Scale to a value beyond the permitted range.