disable modes

src/textual/demo/demo_app.py

@@ -93,3 +93,13 @@ def action_maximize(self) -> None:
                     title="Maximize",
                     severity="warning",
                 )
+
+    def check_action(self, action: str, parameters: tuple[object, ...]) -> bool | None:
+        """Disable switching to a mode we are already on."""
+        if (
+            action == "switch_mode"
+            and parameters
+            and self.current_mode == parameters[0]
+        ):
+            return None
+        return True