I am trying to add functionality to a go.js diagram such that, you can only create links when the shift key is pressed. I can do that fine. But as a consequence, I don't want the user to be able to move the node when the shift key is pressed.

To achieve part 1, I overrode linkingTool.canStart. This achieved that goal admerably.

To achieve part 2, I overrode draggingTool.canStart. This stops the ability to drag nodes but breaks part 1 with an Uncaught TypeError: this.findLinkablePort is not a function.

tool = myDiagram.toolManager.linkingTool
  tool.canStart = function(){
    return event.shiftKey && go.LinkingTool.prototype.canStart.call(tool)
  }

tool = myDiagram.toolManager.draggingTool
  tool.canStart = function(){
    return !event.shiftKey && go.DraggingTool.prototype.canStart.call(tool)
  }

My assumption is that linkingTool uses draggingTool somehow, but I cant find that in the API and I don't know what I can do to fix it. Any ideas as to what is going on?