No matter how a directory is mounted, including with idmap=file and the users groups (local and on the server propely mapped), it is not possible to change the group of a file on a local mounted directory. This is also not possible when changing the group like that on the server with that user id is possible (so no rood/sudo required).
It is however possible to perform the change directly on the server and the change is then visible on the client.
This might be a limitation of the underlying SFTO protocol, but if there would be a way to make use of the group mappings via idmap=file and gidfile to allow changing the group on the client side this would be immensely useful.
No matter how a directory is mounted, including with idmap=file and the users groups (local and on the server propely mapped), it is not possible to change the group of a file on a local mounted directory. This is also not possible when changing the group like that on the server with that user id is possible (so no rood/sudo required).
It is however possible to perform the change directly on the server and the change is then visible on the client.
This might be a limitation of the underlying SFTO protocol, but if there would be a way to make use of the group mappings via idmap=file and gidfile to allow changing the group on the client side this would be immensely useful.