code: def _viewbox_unset(self, viewbox):
          
          self._viewbox = None
          # Disconnect
          viewbox.events.mouse_press.disconnect(self.viewbox_mouse_event)
          viewbox.events.mouse_release.disconnect(self.viewbox_mouse_event)
          viewbox.events.mouse_move.disconnect(self.viewbox_mouse_event)
          viewbox.events.mouse_wheel.disconnect(self.viewbox_mouse_event)
          viewbox.events.resize.disconnect(self.viewbox_resize_event)
docs: Friend method of viewbox to unregister itself.