UX: When summarizing/filtering by participant jump to second post

app/assets/javascripts/discourse/models/post-stream.js.es6

@@ -16,6 +16,7 @@ function calcDayDiff(p1, p2) {
     }
   }
 }
+
 const PostStream = RestModel.extend({
   loading: Em.computed.or('loadingAbove', 'loadingBelow', 'loadingFilter', 'stagingPost'),
   notLoading: Em.computed.not('loading'),
@@ -137,7 +138,13 @@ const PostStream = RestModel.extend({
   toggleSummary() {
     this.get('userFilters').clear();
     this.toggleProperty('summary');
-    return this.refresh();
+
+    const self = this;
+    return this.refresh().then(function() {
+      if (self.get('summary')) {
+        self.jumpToSecondVisible();
+      }
+    });
   },
 
   toggleDeleted() {
@@ -145,17 +152,33 @@ const PostStream = RestModel.extend({
     return this.refresh();
   },
 
+  jumpToSecondVisible() {
+    const posts = this.get('posts');
+    if (posts.length > 1) {
+      const secondPostNum = posts[1].get('post_number');
+      Discourse.URL.jumpToPost(secondPostNum);
+    }
+  },
+
   // Filter the stream to a particular user.
   toggleParticipant(username) {
     const userFilters = this.get('userFilters');
     this.set('summary', false);
     this.set('show_deleted', true);
+
+    let jump = false;
     if (userFilters.contains(username)) {
       userFilters.removeObject(username);
     } else {
       userFilters.addObject(username);
+      jump = true;
     }
-    return this.refresh();
+    const self = this;
+    return this.refresh().then(function() {
+      if (jump) {
+        self.jumpToSecondVisible();
+      }
+    });
   },
 
   /**