If we successfully join, display a spinner until the js-sdk indicates (via room membership event or room event) that we can start using the room normally. A room event indicates we have never seen that room which means we need to use the new room object to clobber state.room. This is to make sure we replace the room that is set up for peeking with the room that can be used normally. For historical rooms, this isn't a problem.
This is a workaround for the fact that when peeking, the js-sdk calls onRoom, which is difficult to handle from the clients perspective because onRoom should only be called for rooms that you've never seen before. But if you peek a room that you've joined and left and get an onRoom, you run into trouble. You also can't just always use onRoomMembership because this won't be triggered for the first time you see the room. So we end up using a combination of both.
See https://github.com/matrix-org/matrix-js-sdk/issues/464 for discussion on improving this
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