feat: check for key mutation when removing children

reacton/core.py

@@ -2129,7 +2129,12 @@ def _remove_element(self, el: Element, default_key: str, parent_key):
                 new_parent_key = join_key(parent_key, key)
                 self._remove_element(self.context.root_element, "/", parent_key=new_parent_key)
             finally:
+                def _check_key_mutation(child_context: ComponentContext):
+                    for key, el in child_context.elements.items():
+                        if key != el._key:
+                            raise RuntimeError(f"Key mismatch: {key} != {el._key}. Did you mutate a key?")
                 try:
+                    _check_key_mutation(child_context)
                     assert not child_context.elements, f"left over elements {child_context.elements}"
                     assert not child_context.element_to_widget, f"left over element_to_widget {child_context.element_to_widget}"
                     assert not child_context.widgets, f"left over widgets {child_context.widgets}"