I have a very odd sitation with object selection.
After further testing. I can add that it is like a “blackspot” on the diagram.
What classes (node, view, tools, etc) have you overriden? What methods within those classes?
Another possibility is that there is an unseen unselectable and/or unmovable object at that region that is getting the mouse events. Might it be a view object rather than a document object?
Iterate over all of the layers of your GoView to see how many objects there really are.