code: def pa_zoom(self, viewer, event, msg=True):
          
          event = self._pa_synth_scroll_event(event)
          if event.state != 'move':
              return False
          self._sc_zoom(viewer, event, msg=msg, origin=None)
          return True
docs: Interactively zoom the image by a pan gesture.
          (the back end must support gestures)