[Epic] Simplified Sliding Sync in Element Web

[dbkr]

New, updated tracker issue for simplified sliding sync as the old one was… old, and referred to regular sliding sync.

js-sdk PR: matrix-org/matrix-js-sdk#4400
web PR: #28515

• Adapt current sliding sync impl to new SSS (PoC version)
• Fix bug where it uses SSS even if the labs flag isn't turned on - I think this may have been intentional on the PR for some reason: to double check
• Get tests passing on SSS PRs
  • js-sdk
  • element-web
• Code coverage on SSS PR
• Look at network requests & unnecessary subscribe / unsubscribe
• Prevent current SS users from breaking: swap to new labs flag, throw error if old labs flag is turned on
• Get SSS PR merged (at this point we have SSS in labs replacing the old SS labs flag. Knocking and presence will not work if the labs flag is enabled).
• Room list takes ages to update properly with full list of rooms
• Get unread dots working (I don't understand why this doesn't work but it doesn't)
• Fix ability to mark a room as read (don't seem to be able to clear room notifications)
• Tombstoned rooms are shown in the room list
• Allow it to be turned off again (this is fine now it's no longer proxy-based)
• Rooms missing from breadcrumbs (eg. it only shows 2, presumably the ones in the first sync response)
• Get pinned messages working (I don't know why they don't work but empirically they do not)
• Events sometimes get put in room at the wrong point!
  • Made a room, invited another client, sent some messages from the other client without opening it on the SSS one (including a threaded message), opened the room on the SSS client: room create event ended up after the messages.
• Get knocking working
  • May require support in synapse
  • How much work is this? - I think this actually works fine after matrix-org/matrix-js-sdk@ec83c14
• Get presence working
  • Require support in synapse and the spec
  • May not be all that much work(?) as it should be similar to the v2 impl, but does not currently exist.
• Make the widget API work, eg. Element Call doesn't get all the state it needs.
• Any other unknown unknowns that need fixing with the SSS experience
• Make SSS auto detect server support and fall back to regular sync v2
• De-labs!
• (probably significantly later) Remove sync v2 support

More detail to come as I explore what's needed for the various parts.

[paboum]

I would suggest starting from this:

The message is confusing as this is Element Web that does not support the current SS protocol (Synapse already has SSS since 1.114). Instead, this could be renamed to SS v1 or legacy SS.

[t3chguy]

@paboum EW supports Sliding Sync, it doesn't support an entirely different MSC called Simplified Sliding Sync, neither is v2 nor legacy. Both are distinct MSCs.