Room scroll state is broken mk2

[greentore]

Steps to reproduce

Follow-up to #25810
Room scroll position isn't properly maintained again since Element v1.11.50 (matrix-react-sdk v3.85.0). I confirmed it works properly in v1.11.49.
Regressed by matrix-org/matrix-react-sdk@01e7e01 / matrix-org/matrix-react-sdk#11799
More specifically, the issue goes away if you comment out dis.dispatch<ActionPayload>({ action: Action.RoomLoaded }); in src/components/structures/RoomView.tsx, but obviously the real bug is somewhere deeper.

Outcome

What did you expect?

The room's scroll position is restored.

What happened instead?

The room's scroll position is restored only to scroll to the bottom a moment later.

Operating system

No response

Browser information

No response

URL for webapp

No response

Application version

No response

Homeserver

No response

Will you send logs?

No

[florianduros]

My analysis:

1. The Allow adding extra icons to the room header matrix-org/matrix-react-sdk#11799 PR added the dispatch of a new event Action.RoomLoaded
2. This event is catched by the RoomViewStore and the RoomViewStore is updated
3. This update triggers an extra call of RoomView#onRoomViewStoreUpdate.
4. However RoomView#onRoomViewStoreUpdate was already called when the RoomView component was mount. The newState.initialEventId field is computed only on the first call of RoomView#onRoomViewStoreUpdate
5. RoomView#state.initialEventId is undefined and the TimelinePanel position is set to the end