FIX: One-by-off error in topic show action (#13183)

The not found condition did not work for topics with chunk_size posts,
because it considered it has two pages, but it only has one.

app/controllers/topics_controller.rb

@@ -128,7 +128,7 @@ class TopicsController < ApplicationController
     end
 
     page = params[:page]
-    if (page < 0) || ((page - 1) * @topic_view.chunk_size > @topic_view.topic.highest_post_number)
+    if (page < 0) || ((page - 1) * @topic_view.chunk_size >= @topic_view.topic.highest_post_number)
       raise Discourse::NotFound
     end
 

spec/requests/topics_controller_spec.rb

@@ -2157,6 +2157,28 @@ RSpec.describe TopicsController do
         get "/t/#{topic.slug}/#{topic.id}/#{post_number}.json"
         expect(response.status).to eq(200)
         expect(extract_post_stream).to eq(@post_ids[-2..-1])
+
+        TopicView.stubs(:chunk_size).returns(3)
+
+        get "/t/#{topic.slug}/#{topic.id}.json", params: { page: 1 }
+        expect(response.status).to eq(200)
+        expect(extract_post_stream).to eq(@post_ids[0..2])
+
+        get "/t/#{topic.slug}/#{topic.id}.json", params: { page: 2 }
+        expect(response.status).to eq(200)
+        expect(extract_post_stream).to eq(@post_ids[3..3])
+
+        get "/t/#{topic.slug}/#{topic.id}.json", params: { page: 3 }
+        expect(response.status).to eq(404)
+
+        TopicView.stubs(:chunk_size).returns(4)
+
+        get "/t/#{topic.slug}/#{topic.id}.json", params: { page: 1 }
+        expect(response.status).to eq(200)
+        expect(extract_post_stream).to eq(@post_ids[0..3])
+
+        get "/t/#{topic.slug}/#{topic.id}.json", params: { page: 2 }
+        expect(response.status).to eq(404)
       end
     end