Function set_showing 

async fn set_showing<S>(
    socket: &mut ClientSocket<S>,
    state: &Arc<TransportState>,
    watching: &mut Watching,
    seat: Option<SessionSlot>,
) -> Result<()>
where
    S: Sink<Message, Error = Error> + Unpin,

Move this connection’s shown seat, spending exactly what that showing earns.

Attach is what clears the diamond and the failure mark, so it follows the screen rather than the subscription: a client keeps every visited seat subscribed, and spending on that would clear marks nobody looked at. The answer is whether anything was spent, which is what the home it holds is owed a re-encode for.