Broken room state cross platform

[BillCarsonFr]

We have now reports on the 3 platforms regarding broken/incomplete states. Leading to several sorts of bad behaviour, going from missing topic, to denying actions (missing PL), and to sending in clear in e2e rooms

IOS: element-hq/element-ios#5184
Web: element-hq/element-web#20435
Android: element-hq/element-android#5068

[ara4n]

some incorrect state could be due to:

• /sync API does not tell clients when the server's view of state changes outside the timeline matrix-org/matrix-spec#1209 (solved in sync v3, unsolved in v2)
• state resets still happen in v2 rooms matrix-org/synapse#8629

However, clients are also at risk of corrupting their own state (e.g. if iOS is failing to play back NSE sync responses correctly).

I suspect there are also situations where synapse presents non-atomic state to clients when it's in the middle of joining a room.