[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.

[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.