Use a fixed limit for mega topic posts count.

lib/topic_view.rb

@@ -378,7 +378,7 @@ class TopicView
 
       columns = [:id, :post_number]
 
-      if is_mega_topic?
+      if !is_mega_topic?
         columns << 'EXTRACT(DAYS FROM CURRENT_TIMESTAMP - created_at)::INT AS days_ago'
       end
 
@@ -563,7 +563,9 @@ class TopicView
     filtered_post_ids.index(closest_post.first) || filtered_post_ids[0]
   end
 
+  MEGA_TOPIC_POSTS_COUNT = 10000
+
   def is_mega_topic?
-    !@topic.private_message? && @topic.posts_count >= SiteSetting.auto_close_topics_post_count
+    @topic.posts_count >= MEGA_TOPIC_POSTS_COUNT
   end
 end

spec/components/topic_view_spec.rb

@@ -565,13 +565,20 @@ describe TopicView do
 
     describe 'for mega topics' do
       it 'should return the right columns' do
-        SiteSetting.auto_close_topics_post_count = 2
+        begin
+          original_const = TopicView::MEGA_TOPIC_POSTS_COUNT
+          TopicView.send(:remove_const, "MEGA_TOPIC_POSTS_COUNT")
+          TopicView.const_set("MEGA_TOPIC_POSTS_COUNT", 2)
 
-        expect(topic_view.filtered_post_stream).to eq([
-          [post.id, post.post_number],
-          [post2.id, post2.post_number],
-          [post3.id, post3.post_number]
-        ])
+          expect(topic_view.filtered_post_stream).to eq([
+            [post.id, post.post_number],
+            [post2.id, post2.post_number],
+            [post3.id, post3.post_number]
+          ])
+        ensure
+          TopicView.send(:remove_const, "MEGA_TOPIC_POSTS_COUNT")
+          TopicView.const_set("MEGA_TOPIC_POSTS_COUNT", original_const)
+        end
       end
     end
   end