correct logic for tracking the highest seen post number so its always consistent

app/assets/javascripts/discourse/components/screen_track.js

@@ -28,12 +28,6 @@ Discourse.ScreenTrack = Ember.Object.extend({
     };
   },
 
-  guessedSeen: function(postNumber) {
-    if (postNumber > (this.highestSeen || 0)) {
-      this.highestSeen = postNumber;
-    }
-  },
-
   // Reset our timers
   reset: function() {
     this.lastTick = new Date().getTime();
@@ -93,16 +87,21 @@ Discourse.ScreenTrack = Ember.Object.extend({
       timing.time = 0;
     });
     topicId = this.get('topic_id');
+
+    var highestSeen = 0;
+    $.each(newTimings, function(postNumber){
+      highestSeen = Math.max(highestSeen, parseInt(postNumber, 10));
+    });
+
     highestSeenByTopic = Discourse.get('highestSeenByTopic');
-    if ((highestSeenByTopic[topicId] || 0) < this.highestSeen) {
-      highestSeenByTopic[topicId] = this.highestSeen;
+    if ((highestSeenByTopic[topicId] || 0) < highestSeen) {
+      highestSeenByTopic[topicId] = highestSeen;
     }
     if (!Object.isEmpty(newTimings)) {
       Discourse.ajax(Discourse.getURL('/topics/timings'), {
         data: {
           timings: newTimings,
           topic_time: this.topicTime,
-          highest_seen: this.highestSeen,
           topic_id: topicId
         },
         cache: false,

app/assets/javascripts/discourse/views/topic_view.js

@@ -181,8 +181,6 @@ Discourse.TopicView = Discourse.View.extend(Discourse.Scrolling, {
       }
       if (!post.get('read')) {
         post.set('read', true);
-        screenTrack = this.get('screenTrack');
-        if (screenTrack) { screenTrack.guessedSeen(postNumber); }
       }
       return post.get('post_number');
     }

app/controllers/topics_controller.rb

@@ -151,7 +151,6 @@ class TopicsController < ApplicationController
     PostTiming.process_timings(
       current_user,
       params[:topic_id].to_i,
-      params[:highest_seen].to_i,
       params[:topic_time].to_i,
       (params[:timings] || []).map{|post_number, t| [post_number.to_i, t.to_i]}
     )

app/models/post_timing.rb

@@ -37,15 +37,19 @@ class PostTiming < ActiveRecord::Base
   end
 
 
-  def self.process_timings(current_user, topic_id, highest_seen, topic_time, timings)
+  def self.process_timings(current_user, topic_id, topic_time, timings)
     current_user.update_time_read!
 
+    highest_seen = 1
     timings.each do |post_number, time|
       if post_number >= 0
         PostTiming.record_timing(topic_id: topic_id,
                                  post_number: post_number,
                                  user_id: current_user.id,
                                  msecs: time)
+
+        highest_seen = post_number.to_i > highest_seen ?
+                       post_number.to_i : highest_seen
       end
     end
 

app/models/topic_user.rb

@@ -117,6 +117,10 @@ class TopicUser < ActiveRecord::Base
         threshold: SiteSetting.auto_track_topics_after
       }
 
+      # In case anyone seens "seen_post_count" and gets confused, like I do.
+      # seen_post_count represents the highest_post_number of the topic when
+      # the user visited it. It may be out of alignement with last_read, meaning
+      # ... user visited the topic but did not read the posts
       rows = exec_sql("UPDATE topic_users
                                     SET
                                       last_read_post_number = greatest(:post_number, tu.last_read_post_number),
@@ -176,17 +180,16 @@ class TopicUser < ActiveRecord::Base
 UPDATE topic_users t
   SET
     last_read_post_number = last_read,
-    seen_post_count = post_count
+    seen_post_count = GREATEST(t.seen_post_count, last_read)
 FROM (
-  SELECT topic_id, user_id, COUNT(*) post_count, MAX(post_number) last_read
+  SELECT topic_id, user_id, MAX(post_number) last_read
   FROM post_timings
   GROUP BY topic_id, user_id
 ) as X
 WHERE X.topic_id = t.topic_id AND
       X.user_id = t.user_id AND
       (
-        last_read_post_number <> last_read OR
-        seen_post_count <> post_count
+        last_read_post_number <> last_read
       )
 SQL
   end

spec/models/post_timing_spec.rb

@@ -21,7 +21,7 @@ describe PostTiming do
 
       post.user.unread_notifications_by_type.should == { Notification.types[:liked] => 1 }
 
-      PostTiming.process_timings(post.user, post.topic_id, 1, 100, [[post.post_number, 100]])
+      PostTiming.process_timings(post.user, post.topic_id, 1, [[post.post_number, 100]])
 
       post.user.reload
       post.user.unread_notifications_by_type.should == {}