@@ -712,6 +712,8 @@ def inputhook(self, context):
     active_eventloop = None
     def enable_gui(self, gui=None):
         print(f'Someone called `enable_gui` with {gui=}.')
+        if self._inputhook is not None and gui is not None:
+            raise RuntimeError("Shell already running a gui event loop.")
         if gui and (gui not in {"inline", "webagg"}):
             # This hook runs with each cycle of the `prompt_toolkit`'s event loop.
             self.active_eventloop, self._inputhook = get_inputhook_name_and_func(gui)