remove computed property for progress bar glyph

app/assets/javascripts/discourse/controllers/topic-progress.js.es6

@@ -40,10 +40,6 @@ export default Ember.ObjectController.extend({
     Discourse.URL.routeTo(url);
   },
 
-  chevronClass: function() {
-    return this.get('expanded') ? '' : 'fa-sort';
-  }.property('expanded'),
-
   streamPercentage: function() {
     if (!this.get('postStream.loaded')) { return 0; }
     if (this.get('postStream.highest_post_number') === 0) { return 0; }

app/assets/javascripts/discourse/templates/topic-progress.js.handlebars

@@ -17,6 +17,6 @@
   <div class='nums'>
     <h4>{{progressPosition}}</h4><span {{bind-attr class="hugeNumberOfPosts:hidden"}}> <span>{{i18n of_value}}</span> <h4>{{postStream.filteredPostsCount}}</h4></span>
   </div>
-  <i {{bind-attr class=":fa chevronClass"}}></i>
+  <i {{bind-attr class=":fa expanded::fa-sort"}}></i>
   <div class='bg'>&nbsp;</div>
 </nav>