FIX: "Go Back" wasn't displaying properly most of the time

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

@@ -849,14 +849,25 @@ export default Ember.Controller.extend(SelectedPostsCount, BufferedContent, {
 
     if (topic.get('id') === topicId) {
 
+      let highestReadPostId = 0;
+
       // TODO identity map for postNumber
       postStream.get('posts').forEach(post => {
         if (!post.read && postNumbers.indexOf(post.post_number) !== -1) {
+          const id = post.get('id');
+          if (id > highestReadPostId) {
+            highestReadPostId = id;
+          }
+
           post.set('read', true);
-          this.appEvents.trigger('post-stream:refresh', { id: post.id });
+          this.appEvents.trigger('post-stream:refresh', { id });
         }
       });
 
+      if (highestReadPostId > 0) {
+        topic.set('last_read_post_id', highestReadPostId);
+      }
+
       const max = _.max(postNumbers);
       if (max > topic.get("last_read_post_number")) {
         topic.set("last_read_post_number", max);

app/assets/javascripts/discourse/models/topic-tracking-state.js.es6

@@ -320,11 +320,6 @@ const TopicTrackingState = Discourse.Model.extend({
             .length;
   },
 
-  lastReadPostNumber(topicId) {
-    const state = this.states[`t${topicId}`];
-    return state ? state.last_read_post_number : null;
-  },
-
   countCategory(category_id) {
     let sum = 0;
     _.each(this.states, function(topic){

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

@@ -20,7 +20,7 @@ createWidget('timeline-last-read', {
       this.attach('button', {
         className: 'btn btn-primary btn-small',
         icon: 'arrow-left',
-        label: 'go_back',
+        label: 'topic.timeline.back',
         action: 'goBack'
       })
     ];
@@ -92,7 +92,8 @@ createWidget('timeline-scrollarea', {
   position() {
     const { attrs } = this;
     const percentage = this.state.percentage;
-    const postStream = attrs.topic.get('postStream');
+    const topic = attrs.topic;
+    const postStream = topic.get('postStream');
     const total = postStream.get('filteredPostsCount');
     let current = Math.round(total * percentage);
 
@@ -111,12 +112,13 @@ createWidget('timeline-scrollarea', {
       lastReadPercentage: null
     };
 
-    if (attrs.topicTrackingState) {
+    const lastReadId = topic.last_read_post_id;
-      const lastRead = attrs.topicTrackingState.lastReadPostNumber(attrs.topic.id);
+    const lastReadNumber = topic.last_read_post_number;
-      if (lastRead) {
+
-        result.lastRead = lastRead;
+    if (lastReadId && lastReadNumber) {
-        result.lastReadPercentage = lastRead === 1 ? 0.0 : parseFloat(lastRead) / total;
+      const idx = postStream.get('stream').indexOf(lastReadId) + 1;
-      }
+      result.lastRead = lastReadNumber;
+      result.lastReadPercentage = this._percentFor(topic, idx);
     }
 
     return result;
@@ -173,7 +175,6 @@ createWidget('timeline-scrollarea', {
 
   _percentFor(topic, postNumber) {
     const total = topic.get('postStream.filteredPostsCount');
-    console.log(postNumber, total);
     let result = (postNumber === 1) ? 0.0 : parseFloat(postNumber) / total;
 
     if (result < 0) { return 0.0; }

app/serializers/topic_view_serializer.rb

@@ -47,6 +47,7 @@ class TopicViewSerializer < ApplicationSerializer
              :details,
              :highest_post_number,
              :last_read_post_number,
+             :last_read_post_id,
              :deleted_by,
              :has_deleted,
              :actions_summary,
@@ -164,8 +165,16 @@ class TopicViewSerializer < ApplicationSerializer
     object.highest_post_number
   end
 
+  def last_read_post_id
+    return nil unless object.filtered_post_stream && last_read_post_number
+    object.filtered_post_stream.each do |ps|
+      return ps[0] if ps[1] === last_read_post_number
+    end
+  end
+  alias_method :include_last_read_post_id?, :has_topic_user?
+
   def last_read_post_number
-    object.topic_user.last_read_post_number
+    @last_read_post_number ||= object.topic_user.last_read_post_number
   end
   alias_method :include_last_read_post_number?, :has_topic_user?
 

config/locales/client.en.yml

@@ -1294,6 +1294,7 @@ en:
       auto_close_immediate: "The last post in the topic is already %{hours} hours old, so the topic will be closed immediately."
 
       timeline:
+        back: "Back"
         replies: "%{current} / %{total} replies"
         replies_short: "%{current} / %{total}"