History sharing: UX for telling users that history may be shared

[richvdh]

Algorithm:

• Whenever the user opens an encrypted room with history_visibility != joined, show the user a warning banner, unless the flag showing it has been dismissed is set. If the user dismisses the banner, set a flag in the client's local db recording that it has been dismissed for that room.
• Whenever the user opens an encrypted room with history_visibility == joined, clear the dismissed flag, if it is set. This ensures that if a room is changed public -> private -> public, we show the banner again when it is set back to public.

Clients:

• Web: Implement UI for history visibility acknowledgement. element-web#31156
• Android: Add alert to encrypted rooms with visible history (Android). element-x-android#5709
• iOS: Add alert to encrypted rooms with visible history (iOS). element-x-ios#4738

[richvdh]

Designs can be found at the figma links on #2829

[mxandreas]

@richvdh

Whenever the user opens an encrypted room with history_visibility != joined, show the user a warning banner

Note that most rooms currently use invited not joined (our clients do not use joined at all - there's no good use case for that). Thus, I think we should check here that history_visibility is not in (joined, invited) and likewise for the other expression.

[mxandreas]

Also, as per our discussion, please add to the conditions that we do not show the banner in DMs, regardless of what the actual history visibility is, because we would never share keys for history when invited to a DM.

[richvdh]

because we would never share keys for history when invited to a DM.

That's not the case. What we could do is, when we send messages in a DM, set the shared_history flag to false even if history_visibility is set to shared. That would need implementation, though.

[mxandreas]

That's not the case.

I meant this as an overarching goal (e.g. effectively a user who joins a DM will not get a key for history, ever). What's the best way to accomplish that, I do not have a strong opinion. But I guess it needs to be done because of how the DMs are set up to today in terms of history sharing.