As a user I don't want to see UISIs

[lampholder]

As much as possible:

• if there's a "good reason" I can't decrypt the message, tell me what it is
• if there's something I can do to resolve the issue, present to me the option

[lampholder]

We should consider getting rid of Members only (since the point in time of selecting this option) in rooms which are encrypted by default to minimise "you weren't in the room yet" UISIs.

[ara4n]

Causes of seeing UISIs:

• Your device has been blocked (As a user, I expect to know if my device has been blocked rather than seeing UISIs #31)
• The messages predate your entry to the room
• You weren't logged into any devices when the messages were sent (As a user I expect to be able to read messages in encrypted rooms without worrying about whether I had logged-in devices when they were sent #25)
• The sender didn't know your device was in the room (because you added it during a netsplit)
• The sender didn't know you were in the room (because you joined during a netsplit)
• You have run out of One Time Keys (OTKs) published on the server - either because you've been idle or because someone DoSed you by consuming them all.
• You've logged in on a new device and you haven't restored your keys from backup (offline or online backup), or the backup is incomplete
• You backfilled encrypted messages from an intermediary server, but the origin server can't talk to you to send you the keys.
• You added a device to your account after being invited to a room, but before joining that room - so servers in that room don't know about the new device.

Classes of bugs causing UISIs:

• Olm sessions are wedged due to devices getting out of sync or mangled messages, and we fail to recognise this and start a new session.
• Your server's device list cache is out of sync with reality so you're not encrypting for the right devices
• Your client's device list is out of sync with reality so you're not encrypting for the right devices.