Fix the race condition.

ReactiveFeedback/SignalProducer+System.swift

@@ -29,8 +29,23 @@ extension SignalProducer where Error == NoError {
 
             return SignalProducer<Event, NoError>(Signal.merge(events))
                 .scan(initial, reduce)
-                .prefix(value: initial)
                 .on(value: stateObserver.send(value:))
+                .prefix(value: initial)
+                .on(started: {
+                    // NOTE: Due to the nature of `prefix` lazily starting the producer being prefixed, we cannot rely
+                    //       on `on(value:)` to ignite the feedbacks with the initial state.
+                    //
+                    //       At the time `prefix(value:)` calls `on(value:)` for the initial value, the events-reducer
+                    //       producer has not yet been started yet. Consequentially, it would lead to dropped events
+                    //       when the system is instantiated on a queue different from the queue used for
+                    //       serializing events.
+                    //
+                    //       Having said that, `prefix(value:)` is guaranteed to have started the prefixed producer as
+                    //       part of the synchronous producer starting process. So we can addres the issue by applying
+                    //       `on(started:)` after `prefix(value:)` to ignite the system, while having `on(value:)`
+                    //       instead applied before `prefix(value:)` to keep the reducer-to-feedbacks path open.
+                    stateObserver.send(value: initial)
+                })
         }
     }