docs: def execution_engine_model_changed(self, model, prop_name, info):
          """High light active state machine. """
  
          notebook = self.view['notebook']
          active_state_machine_id = self.model.state_machine_manager.active_state_machine_id
  
          if active_state_machine_id is None:
              # un-mark all state machine that are marked with execution-running style class
              for tab in self.tabs.values():
                  label = notebook.get_tab_label(tab['page']).get_child().get_children()[0]
                  if label.get_style_context().has_class(constants.execution_running_style_class):
                      label.get_style_context().remove_class(constants.execution_running_style_class)
          else:
              # mark active state machine with execution-running style class
              page = self.get_page_for_state_machine_id(active_state_machine_id)
              if page:
                  label = notebook.get_tab_label(page).get_child().get_children()[0]
                  label.get_style_context().add_class(constants.execution_running_style_class)