Element-web makes invalid assumptions about room membership changes with lazy-loading

[richvdh]

Consider:

• Alice joins a room and gets an initial /sync response in that room. Due to lazy-loading, the membership list is incomplete.
• We decide to load the full membership list (eg, Alice opens the user list, or sends an encrypted message).
• Alice closes the tab for a while.
• Bob joins the room and sends a bunch of messages.
• Alice opens the tab again, and does an incremental /sync; she gets a gappy /sync result.
• Alice sends an encrypted message.

Now, the problem is that we do not refresh the membership list on the gappy sync result. The spec for lazy-loading is unclear about exactly what assumptions we can make, but it seems that, whilst Synapse disables lazy-loading for incremental syncs, Conduit does not.

I'm filing this here as the net effect is that, when used against Conduit, Element-Web will send undecryptable events. It's to be determined whether this should be considered a bug in Element or Conduit.

[erikjohnston]

Note that this will be an issue with simplified sliding sync, where we will not all membership updates in incremental sync. We need to hammer down the exact semantics, but it may be something like "we don't send all membership updates for gappy syncs".