UX: order categories based on recent activity when using categories_and_latest_topics layout (#7166)

app/models/category_list.rb

@@ -77,6 +77,8 @@ class CategoryList
 
     if SiteSetting.fixed_category_positions
       @categories = @categories.order(:position, :id)
+    elsif !SiteSetting.fixed_category_positions && SiteSetting.desktop_category_page_style == "categories_and_latest_topics"
+      @categories = @categories.includes(:latest_post).order("posts.created_at DESC NULLS LAST").order('categories.id ASC')
     else
       @categories = @categories.order('COALESCE(categories.posts_week, 0) DESC')
         .order('COALESCE(categories.posts_month, 0) DESC')

spec/models/category_list_spec.rb

@@ -147,6 +147,7 @@ describe CategoryList do
     context 'fixed_category_positions is disabled' do
       before do
         SiteSetting.fixed_category_positions = false
+        SiteSetting.desktop_category_page_style = "categories_and_top_topics"
       end
 
       it "returns categories in order of activity" do
@@ -159,6 +160,22 @@ describe CategoryList do
         cat1, cat2 = Fabricate(:category, position: 1), Fabricate(:category, position: 0)
         expect(category_ids).to eq([cat1.id, cat2.id])
       end
+
+      context "when using categories_and_latest_topics layout" do
+        before do
+          SiteSetting.desktop_category_page_style = "categories_and_latest_topics"
+        end
+
+        it "returns categories in order of latest activity" do
+          post1 = Fabricate(:post, created_at: 1.hour.ago)
+          post2 = Fabricate(:post, created_at: 1.day.ago)
+          post3 = Fabricate(:post, created_at: 1.week.ago)
+          cat1 = Fabricate(:category, position: 0, latest_post_id: post2.id)
+          cat2 = Fabricate(:category, position: 1, latest_post_id: post3.id)
+          cat3 = Fabricate(:category, position: 1, latest_post_id: post1.id)
+          expect(category_ids).to eq([cat3.id, cat1.id, cat2.id])
+        end
+      end
     end
   end