DEV: Better categories pagination (#26421)

Pagination is enabled only when "lazy load categories" is enabled. For
those cases when it is not, the first page should return all the
results.

app/models/category_list.rb

@@ -142,13 +142,17 @@ class CategoryList
 
     query = self.class.order_categories(query)
 
+    page = [1, @options[:page].to_i].max
     if @guardian.can_lazy_load_categories? && @options[:parent_category_id].blank?
-      page = [1, @options[:page].to_i].max
       query =
         query
           .where(parent_category_id: nil)
           .limit(CATEGORIES_PER_PAGE)
           .offset((page - 1) * CATEGORIES_PER_PAGE)
+    elsif page > 1
+      # Pagination is supported only when lazy load is enabled. If it is not,
+      # everything is returned on page 1.
+      query = query.none
     end
 
     query =

spec/requests/categories_controller_spec.rb

@@ -397,6 +397,35 @@ RSpec.describe CategoriesController do
         response.parsed_body["category_list"]["categories"].map { |x| x["id"] },
       ).not_to include(uncategorized.id)
     end
+
+    describe "with page" do
+      before { sign_in(admin) }
+
+      let!(:category2) { Fabricate(:category, user: admin) }
+      let!(:category3) { Fabricate(:category, user: admin) }
+
+      it "paginates results wihen lazy_load_categories is enabled" do
+        SiteSetting.lazy_load_categories_groups = "#{Group::AUTO_GROUPS[:everyone]}"
+
+        stub_const(CategoryList, "CATEGORIES_PER_PAGE", 2) { get "/categories.json?page=1" }
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["category_list"]["categories"].count).to eq(2)
+
+        stub_const(CategoryList, "CATEGORIES_PER_PAGE", 2) { get "/categories.json?page=2" }
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["category_list"]["categories"].count).to eq(2)
+      end
+
+      it "does not paginate results when lazy_load_categories is disabled" do
+        stub_const(CategoryList, "CATEGORIES_PER_PAGE", 2) { get "/categories.json?page=1" }
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["category_list"]["categories"].count).to eq(4)
+
+        stub_const(CategoryList, "CATEGORIES_PER_PAGE", 2) { get "/categories.json?page=2" }
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["category_list"]["categories"].count).to eq(0)
+      end
+    end
   end
 
   describe "extensibility event" do