FIX: More incorrect scrollbar positions on the timeline

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

@@ -8,6 +8,7 @@ export default MountWidget.extend(Docking, {
   buildArgs() {
     return { topic: this.get('topic'),
              topicTrackingState: this.topicTrackingState,
+             enteredIndex: this.get('enteredIndex'),
              dockAt: this.dockAt };
   },
 

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

@@ -19,6 +19,7 @@ export default Ember.Controller.extend(SelectedPostsCount, BufferedContent, {
   queryParams: ['filter', 'username_filters', 'show_deleted'],
   loadedAllPosts: Ember.computed.or('model.postStream.loadedAllPosts', 'model.postStream.loadingLastPost'),
   enteredAt: null,
+  enteredIndex: null,
   retrying: false,
 
   topicDelegated: [

app/assets/javascripts/discourse/routes/topic-from-params.js.es6

@@ -32,6 +32,7 @@ export default Discourse.Route.extend({
 
       topicController.setProperties({
         'model.currentPost': closest,
+        enteredIndex: postStream.get('stream').indexOf(closestPost.get('id')),
         enteredAt: new Date().getTime().toString(),
       });
 

app/assets/javascripts/discourse/templates/topic.hbs

@@ -72,7 +72,10 @@
         <div class="posts-wrapper">
 
           {{#if showTimeline}}
-            {{topic-timeline topic=model loading=model.postStream.loading delegated=topicDelegated}}
+            {{topic-timeline topic=model
+                             enteredIndex=enteredIndex
+                             loading=model.postStream.loading
+                             delegated=topicDelegated}}
           {{else}}
             {{topic-progress topic=model delegated=topicDelegated}}
             {{topic-admin-menu-button topic=model fixed="true" delegated=topicDelegated}}

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

@@ -87,7 +87,7 @@ createWidget('timeline-scrollarea', {
   },
 
   defaultState(attrs) {
-    return { percentage: this._percentFor(attrs.topic, attrs.topic.currentPost), scrolledPost: 1 };
+    return { percentage: this._percentFor(attrs.topic, attrs.enteredIndex + 1), scrolledPost: 1 };
   },
 
   position() {
@@ -174,7 +174,12 @@ createWidget('timeline-scrollarea', {
 
   _percentFor(topic, postNumber) {
     const total = topic.get('postStream.filteredPostsCount');
-    return postNumber === 1 ? 0.0 : parseFloat(postNumber) / total;
+    console.log(postNumber, total);
+    let result = (postNumber === 1) ? 0.0 : parseFloat(postNumber) / total;
+
+    if (result < 0) { return 0.0; }
+    if (result > 1.0) { return 1.0; }
+    return result;
   }
 });