diff --git a/src/utilities/observables/Concast.ts b/src/utilities/observables/Concast.ts
index 326ae423a1e..5efb77a70f2 100644
--- a/src/utilities/observables/Concast.ts
+++ b/src/utilities/observables/Concast.ts
@@ -47,6 +47,11 @@ export class Concast<T> extends Observable<T> {
   // recent message, though possibly at different times in the past.
   private observers = new Set<Observer<T>>();
 
+  // Shadow observers awaiting broadcast messages. These observers are
+  // subordinated to this.observers, so should not prevent unsubscribing
+  // from this.sub.
+  private shadowObservers = new Set<Observer<T>>();
+
   // This property starts off undefined to indicate the initial
   // subscription has not yet begun, then points to each source
   // subscription in turn, and finally becomes null after the sources have
@@ -95,25 +100,29 @@ export class Concast<T> extends Observable<T> {
     this.handlers.complete!();
   }
 
+  private deliverLastMessage (observer: Observer<T>) {
+    if (this.latest) {
+      const nextOrError = this.latest[0];
+      const method = observer[nextOrError];
+      if (method) {
+        method.call(observer, this.latest[1]);
+      }
+      // If the subscription is already closed, and the last message was
+      // a 'next' message, simulate delivery of the final 'complete'
+      // message again.
+      if (this.sub === null &&
+          nextOrError === "next" &&
+          observer.complete) {
+        observer.complete();
+      }
+    }
+  }
+
   public addObserver(observer: Observer<T>) {
     if (!this.observers.has(observer)) {
       // Immediately deliver the most recent message, so we can always
       // be sure all observers have the latest information.
-      if (this.latest) {
-        const nextOrError = this.latest[0];
-        const method = observer[nextOrError];
-        if (method) {
-          method.call(observer, this.latest[1]);
-        }
-        // If the subscription is already closed, and the last message was
-        // a 'next' message, simulate delivery of the final 'complete'
-        // message again.
-        if (this.sub === null &&
-            nextOrError === "next" &&
-            observer.complete) {
-          observer.complete();
-        }
-      }
+      this.deliverLastMessage(observer);
       this.observers.add(observer);
     }
   }
@@ -156,6 +165,7 @@ export class Concast<T> extends Observable<T> {
       if (this.sub !== null) {
         this.latest = ["next", result];
         iterateObserversSafely(this.observers, "next", result);
+        iterateObserversSafely(this.shadowObservers, "next", result);
       }
     },
 
@@ -166,6 +176,7 @@ export class Concast<T> extends Observable<T> {
         this.latest = ["error", error];
         this.reject(error);
         iterateObserversSafely(this.observers, "error", error);
+        iterateObserversSafely(this.shadowObservers, "error", error);
       }
     },
 
@@ -187,6 +198,7 @@ export class Concast<T> extends Observable<T> {
           // 'next' message (unless there was an error) immediately
           // followed by a 'complete' message (see addObserver).
           iterateObserversSafely(this.observers, "complete");
+          iterateObserversSafely(this.shadowObservers, "complete");
         } else if (isPromiseLike(value)) {
           value.then(obs => this.sub = obs.subscribe(this.handlers));
         } else {
@@ -201,7 +213,7 @@ export class Concast<T> extends Observable<T> {
     const once = () => {
       if (!called) {
         called = true;
-        this.observers.delete(observer);
+        this.shadowObservers.delete(observer);
         callback();
       }
     }
@@ -210,7 +222,11 @@ export class Concast<T> extends Observable<T> {
       error: once,
       complete: once,
     };
-    this.addObserver(observer);
+
+    // Immediately deliver the most recent message, so we can always
+    // be sure all observers have the latest information.
+    this.deliverLastMessage(observer);
+    this.shadowObservers.add(observer);
   }
 
   // A public way to abort observation and broadcast.