BUGFIX: Avoid re-entrency issues with EventEmitter (#2397)

Subscribing or unsubscribing from within an event handler (possibly
transitively) could cause issues or even infinite loops.

This was initially fixed in #2257, which exhibited such problems.
Currently no code is known to have this issue, but it could easily come
up again in the future.

src/utils/EventEmitter.ts

@@ -11,7 +11,11 @@ export class EventEmitter<T extends any[]> {
   }
 
   emit(...args: [...T]): void {
-    for (const sub of this.subscribers) {
+    // It is necessary to make a copy of the subscribers list, because since
+    // the subscribers call arbitrary code, it can eventually call back in and
+    // subscribe or unsubscribe new listeners. We must only dispatch to the
+    // ones that were active at the time the event came in.
+    for (const sub of [...this.subscribers]) {
       sub(...args);
     }
   }