FIX: `/filter` route input field not updating on route change (#23119)

What is the problem here?

When transiting between `/filter` routes with different `q` query
params, the input field is not updating to include the values in the `q`
query param. This was because we were setting the value of the input
field in the constructor of the controller but controllers are actually
singletons in Ember so setting the value of the input field is only done
once when the controller is initialised.

What is the fix here?

Instead of setting the value of the input field in the controller, we
set the value in the `setupController` hook in the route file.

app/assets/javascripts/discourse/app/controllers/navigation/filter.js

@@ -11,11 +11,6 @@ export default class NavigationFilterController extends Controller {
   @tracked copyClass = "btn-default";
   @tracked newQueryString = "";
 
-  constructor() {
-    super(...arguments);
-    this.newQueryString = this.discoveryFilter.q;
-  }
-
   @bind
   updateQueryString(string) {
     this.newQueryString = string;

app/assets/javascripts/discourse/app/routes/discovery-filter.js

@@ -22,6 +22,10 @@ export default class DiscoveryFilterRoute extends DiscourseRoute {
 
   setupController(_controller, model) {
     this.controllerFor("discovery/topics").setProperties({ model });
+
+    this.controllerFor("navigation/filter").setProperties({
+      newQueryString: this.paramsFor("discovery.filter").q,
+    });
   }
 
   renderTemplate() {

spec/system/filtering_topics_spec.rb

@@ -4,9 +4,45 @@ describe "Filtering topics", type: :system do
   fab!(:user) { Fabricate(:user) }
   let(:topic_list) { PageObjects::Components::TopicList.new }
   let(:topic_query_filter) { PageObjects::Components::TopicQueryFilter.new }
+  let(:sidebar) { PageObjects::Components::NavigationMenu::Sidebar.new }
 
   before { SiteSetting.experimental_topics_filter = true }
 
+  it "updates the input field when the query string is changed" do
+    sidebar_section = Fabricate(:sidebar_section, user: user)
+
+    sidebar_section_link_1 =
+      Fabricate(
+        :sidebar_section_link,
+        sidebar_section: sidebar_section,
+        linkable: Fabricate(:sidebar_url, name: "filter tags", value: "/filter?q=tag%3Atag1"),
+      )
+
+    sidebar_section_link_2 =
+      Fabricate(
+        :sidebar_section_link,
+        sidebar_section: sidebar_section,
+        linkable:
+          Fabricate(
+            :sidebar_url,
+            name: "filter categories",
+            value: "/filter?q=category%3Acategory1",
+          ),
+      )
+
+    sign_in(user)
+
+    visit("/latest")
+
+    sidebar.click_section_link("filter tags")
+
+    expect(topic_query_filter).to have_input_text("tag:tag1")
+
+    sidebar.click_section_link("filter categories")
+
+    expect(topic_query_filter).to have_input_text("category:category1")
+  end
+
   describe "when filtering by status" do
     fab!(:topic) { Fabricate(:topic) }
     fab!(:closed_topic) { Fabricate(:topic, closed: true) }

spec/system/page_objects/components/topic_query_filter.rb

@@ -6,6 +6,10 @@ module PageObjects
       def fill_in(text)
         page.fill_in(class: "topic-query-filter__filter-term", with: "#{text}\n")
       end
+
+      def has_input_text?(text)
+        page.has_field?(class: "topic-query-filter__filter-term", with: text)
+      end
     end
   end
 end