FIX: going from /categories to /latest on mobile might break infinite scrolling

app/controllers/categories_controller.rb

@@ -48,11 +48,13 @@ class CategoriesController < ApplicationController
 
         if style == "categories_and_latest_topics".freeze
           @topic_list = TopicQuery.new(current_user, topic_options).list_latest
+          @topic_list.more_topics_url = url_for(public_send("latest_path"))
         elsif style == "categories_and_top_topics".freeze
           @topic_list = TopicQuery.new(nil, topic_options).list_top_for(SiteSetting.top_page_default_timeframe.to_sym)
+          @topic_list.more_topics_url = url_for(public_send("top_path"))
         end
 
-        if @topic_list.present?
+        if @topic_list.present? && @topic_list.topics.present?
           store_preloaded(
             @topic_list.preload_key,
             MultiJson.dump(TopicListSerializer.new(@topic_list, scope: guardian))

lib/topic_query.rb

@@ -401,7 +401,7 @@ class TopicQuery
     end
 
     list = TopicList.new(filter, @user, topics, options.merge(@options))
-    list.per_page = per_page_setting
+    list.per_page = options[:per_page] || per_page_setting
     list
   end
 

spec/requests/categories_controller_spec.rb

@@ -26,6 +26,13 @@ describe CategoriesController do
       expect(html.css('body.crawler')).to be_present
       expect(html.css("a[href=\"/forum/c/#{category.slug}\"]")).to be_present
     end
+
+    it "properly preloads topic list" do
+      SiteSetting.categories_topics = 5
+      SiteSetting.categories_topics.times { Fabricate(:topic) }
+      get "/categories"
+      expect(response.body).to include(%{"more_topics_url":"/latest"})
+    end
   end
 
   context 'extensibility event' do