FIX: Use observer to rerender widget instead.

app/assets/javascripts/discourse/components/topic-notifications-button.js.es6

@@ -9,14 +9,11 @@ export default MountWidget.extend({
   },
 
   @observes('topic.details.notification_level')
-  _triggerEvent() {
-    this.appEvents.trigger('topic-notifications-button:changed', {
-      type: 'notification', id: this.get('topic.details.notification_level')
-    });
+  _queueRerender() {
+    this.queueRerender();
   },
 
   didInsertElement() {
     this._super();
-    this.dispatch('topic-notifications-button:changed', 'topic-notifications-button');
   }
 });

app/assets/javascripts/discourse/components/topic-timeline.js.es6

@@ -38,6 +38,11 @@ export default MountWidget.extend(Docking, {
     this.queueRerender(() => this.queueDockCheck());
   },
 
+  @observes('topic.details.notification_level')
+  _queueRerender() {
+    this.queueRerender();
+  },
+
   dockCheck(info) {
     const mainOffset = $('#main').offset();
     const offsetTop = mainOffset ? mainOffset.top : 0;

app/assets/javascripts/discourse/widgets/topic-notifications-button.js.es6

@@ -91,9 +91,7 @@ export default createWidget('topic-notifications-button', {
   topicNotificationsButtonChanged(msg) {
     switch(msg.type) {
       case 'notification':
-        if (this.attrs.topic.get('details.notification_level') !== msg.id) {
-          this.notificationLevelChanged(msg.id);
-        }
+        this.notificationLevelChanged(msg.id);
         break;
     }
   }