Document read-only mode

Problem

In the context of real-time collaboration, there is a need to have a document appear as read-only, i.e. edits from the UI don't have any effect, but still "live" in the sense that changes made by other users are seen.

Proposed Solution

I'm not sure at which level this should be implemented. For instance, a notebook UI can have several parts that accept changes: cells, graphical elements of a widget... This seems like a lot of places in which to disable editing. And for documents that are opened by an extension, this would leave it to the author to implement read-only mode, so it's not ideal. I'm wondering if there is a more central place in JupyterLab where changes go through, that would make it easier to control them? I'm thinking that a way could be to undo a change from the backend if it comes from a particular user. But it's not really disabling changes, just undoing them afterwards, which will probably be seen in the UI because of the network round-trip. And widgets that are not part of the shared document wouldn't be controllable this way.

Maybe I'm missing something? Curious to hear about other solutions.