docs: def on_widget__button_press_event(self, widget, event):
          '''
          Called when any mouse button is pressed.
  
  
          .. versionchanged:: 0.11
              Do not trigger `route-electrode-added` event if `ALT` key is
              pressed.
          '''
          if self.mode == 'register_video' and event.button == 1:
              self.start_event = event.copy()
              return
          elif self.mode == 'control':
              shape = self.canvas.find_shape(event.x, event.y)
  
              if shape is None: return
              state = event.get_state()
              if event.button == 1:
                  # Start a new route.
                  self._route = Route(self.device)
                  self._route.append(shape)
                  self.last_pressed = shape
                  if not (state & gtk.gdk.MOD1_MASK):
                      # `<Alt>` key is not held down.
                      self.emit('route-electrode-added', shape)