FIX: Make edit categories sidebar modal work more intuitively (#27111)

* Load search results in displayed order so that when more categories are loaded on scroll, they appear at the end,
 * Limit the number of subcategories that are shown per category and display 'show more' links,

spec/requests/categories_controller_spec.rb

@@ -1443,4 +1443,31 @@ RSpec.describe CategoriesController do
       expect(response.parsed_body["categories"].map { |c| c["id"] }).not_to include(category.id)
     end
   end
+
+  describe "#hierachical_search" do
+    before { sign_in(user) }
+
+    it "produces categories with an empty term" do
+      get "/categories/hierarchical_search.json", params: { term: "" }
+
+      expect(response.status).to eq(200)
+      expect(response.parsed_body["categories"].length).not_to eq(0)
+    end
+
+    it "doesn't produce categories with a very specific term" do
+      get "/categories/hierarchical_search.json", params: { term: "acategorythatdoesnotexist" }
+
+      expect(response.status).to eq(200)
+      expect(response.parsed_body["categories"].length).to eq(0)
+    end
+
+    it "doesn't expose secret categories" do
+      category.update!(read_restricted: true)
+
+      get "/categories/hierarchical_search.json", params: { term: "" }
+
+      expect(response.status).to eq(200)
+      expect(response.parsed_body["categories"].map { |c| c["id"] }).not_to include(category.id)
+    end
+  end
 end