In terms of what it does JSON API says the following:

200 OK
If a server accepts an update but also changes the resource(s) in ways other than those specified by the request (for example, updating the updated-at attribute or a computed sha), it MUST return a 200 OK response. The response document MUST include a representation of the updated resource(s) as if a GET request was made to the request URL.

A server MUST return a 200 OK status code if an update is successful, the client’s current fields remain up to date, and the server responds only with top-level meta data. In this case the server MUST NOT include a representation of the updated resource(s).

204 No Content
If an update is successful and the server doesn’t update any fields besides those provided, the server MUST return either a 200 OK status code and response document (as described above) or a 204 No Content status code with no response document.

So I think if we decide to take the convention that if the document doesn't change we do return the 200 with a body rather than try and do the 204 this should be fine and ready for testing.

closing this 1 better to have 1 pr only for the same file