So to recap the need associated with this issue.
For users who do not have access to websockets this is the desired behaviour :
If the user is the only editor of his doc the he can save its document to the server.
If not a banner is displayed explaining his config doesn't allow him to collaborate.

When a user opens a document he has been invited on he cannot edit the document and save new versions to the server even if he has been invited as an editor.

@sampaccoud are you ok with what's described above ? If yes @rl-83 can you provide us with a design ?

Linked with #456

Here is my updated proposal.
The idea is to completely block editing functionality for users who do not have WebSockets.
Clicking on "learn more" opens a modal that explains in simple terms why it isn't working. In this explanatory text, I suggest mentioning: (1) that the user typically has editing rights on this specific document, but (2) that their network configuration does not allow collaboration, and therefore editing.

-> Waiting for the definitive text to update the prototype

This is the text for the modal:
"The network configuration of your workstation or internet access does not allow collaborative editing. Docs uses WebSockets for real-time co-editing. WebSockets are a channel that enables a website and a server to exchange messages instantly and bidirectionally. You can contact your IT administrators to request that WebSockets be allowed."

"La configuration réseau de votre poste de travail ou de votre accès internet de ne permet pas d'utiliser l'édition collaborative.
Docs utilise les websockets la co-édition en temps réel. Les websockets est un canal qui permet à un site web et à un serveur d'échanger des messages instantanément et de façon bidirectionnel. Vous pouvez contacter à vos responsables informatiques pour leur demander d'autoriser les websockets."

@sampaccoud is this ok for you ? if yes we'll start the dev

Small grammatical corrections:

EN
The network configuration of your workstation or internet connection does not allow you to edit shared documents.
Docs uses WebSockets—a communication channel that enables instant, two-way message exchange between your browser and our servers—for real-time editing. To enable collaborative editing, please contact your IT administrators to allow WebSocket connections.

FR
La configuration réseau de votre poste de travail ou de votre connexion internet ne permet pas d’éditer des documents partagés.
Docs utilise les WebSockets pour permettre l'édition en temps réel. Ces canaux de communication permettent des échanges instantanés et bidirectionnels entre votre navigateur et nos serveurs. Pour accéder à l'édition collaborative, veuillez contacter votre service informatique afin d'autoriser les WebSockets.

-> It looks like this

Update: I specified that it was the shared documents that were not editable.

To summarize, user will switch to "Reader" mode if :

• The user is online (has internet)
• The user is not connected to the collaboration server
• The doc has members or is "public editable" or "auth editable"

The following sentence is wrong: "Pour accéder à l'édition collaborative, veuillez contacter votre service informatique afin d'autoriser les WebSockets."

My suggestion: "Pour accéder à l'édition collaborative, veuillez demander à votre service informatique de vous autoriser les WebSockets vers ${HOSTNAME_SERVER}."

To summarize, user will switch to "Reader" mode if :

• The user is online (has internet)
• The user is not connected to the collaboration server
• The doc has members or is "public editable" or "auth editable"

Could we add the condition that at least one other user is currently connected? This would minimize occurrences of locking the document for users with no websocket.

Could we add the condition that at least one other user is currently connected? This would minimize occurrences of locking the document for users with no websocket.

Yes we can know the connected users on the server, but we have a case where multiple users not connected on the server are editing at the same time to tackle as well so. It will require a mechanism to add and remove their presence manually to the collab server (http endpoint).

"a mechanism to add and remove their presence manually to the collab server (http endpoint)": this seems a wise locking solution, for users short of websockets.