attached -> mounted

Author: willmcgugan

src/textual/dom.py

@@ -1581,9 +1581,9 @@ def set_class(self, add: bool, *class_names: str, update: bool = True) -> Self:
             Self.
         """
         if add:
-            self.add_class(*class_names, update=update and self.is_attached)
+            self.add_class(*class_names, update=update and self._is_mounted)
         else:
-            self.remove_class(*class_names, update=update and self.is_attached)
+            self.remove_class(*class_names, update=update and self._is_mounted)
         return self
 
     def set_classes(self, classes: str | Iterable[str]) -> Self: