FIX: Improve topic/header integration when navigating away (#28040)

- Ensure main title is set as 'not visible' when removed from DOM

- `deactivate` -> `willTransition` to ensure proper behavior when navigating between multiple topics

Followup to bdec564d1417c51ef6958784fdd6375fd05da4fc

app/assets/javascripts/discourse/app/components/topic-title.gjs

@@ -3,6 +3,7 @@ import { hash } from "@ember/helper";
 import { on } from "@ember/modifier";
 import { action } from "@ember/object";
 import didInsert from "@ember/render-modifiers/modifiers/did-insert";
+import willDestroy from "@ember/render-modifiers/modifiers/will-destroy";
 import { service } from "@ember/service";
 import PluginOutlet from "discourse/components/plugin-outlet";
 import { isiPad } from "discourse/lib/utilities";
@@ -53,12 +54,26 @@ export default class TopicTitle extends Component {
     }
   }
 
+  @action
+  handleIntersectionChange(e) {
+    // Title is in the viewport or  below it – unusual, but can be caused by
+    // small viewport and/or large headers. Treat same as if title is on screen.
+    this.header.mainTopicTitleVisible =
+      e.isIntersecting || e.boundingClientRect.top > 0;
+  }
+
+  @action
+  handleTitleDestroy() {
+    this.header.mainTopicTitleVisible = false;
+  }
+
   <template>
     {{! template-lint-disable no-invalid-interactive }}
     <div
       {{didInsert this.applyDecorators}}
       {{on "keydown" this.keyDown}}
-      {{observeIntersection this.header.titleIntersectionChanged}}
+      {{observeIntersection this.handleIntersectionChange}}
+      {{willDestroy this.handleTitleDestroy}}
       id="topic-title"
       class="container"
     >

app/assets/javascripts/discourse/app/routes/topic-from-params.js

@@ -52,7 +52,6 @@ export default class TopicFromParams extends DiscourseRoute {
   deactivate() {
     super.deactivate(...arguments);
     this.controllerFor("topic").unsubscribe();
-    this.header.clearTopic();
   }
 
   setupController(controller, params, { _discourse_anchor }) {
@@ -122,9 +121,15 @@ export default class TopicFromParams extends DiscourseRoute {
   }
 
   @action
-  willTransition() {
+  willTransition(transition) {
     this.controllerFor("topic").set("previousURL", document.location.pathname);
 
+    transition.followRedirects().finally(() => {
+      if (!this.router.currentRouteName.startsWith("topic.")) {
+        this.header.clearTopic();
+      }
+    });
+
     // NOTE: omitting this return can break the back button when transitioning quickly between
     // topics and the latest page.
     return true;

app/assets/javascripts/discourse/app/services/header.js

@@ -1,6 +1,5 @@
 import { tracked } from "@glimmer/tracking";
 import { registerDestructor } from "@ember/destroyable";
-import { action } from "@ember/object";
 import { dependentKeyCompat } from "@ember/object/compat";
 import Service, { service } from "@ember/service";
 import { TrackedMap } from "@ember-compat/tracked-built-ins";
@@ -129,22 +128,6 @@ export default class Header extends Service {
     return Array.from(buttonsToHide);
   }
 
-  /**
-   * Called by a modifier attached to the main topic title element.
-   */
-  @action
-  titleIntersectionChanged(e) {
-    if (e.isIntersecting) {
-      this.mainTopicTitleVisible = true;
-    } else if (e.boundingClientRect.top > 0) {
-      // Title is below the curent viewport position. Unusual, but can be caused with
-      // small viewport and/or large headers. Treat same as if title is on screen.
-      this.mainTopicTitleVisible = true;
-    } else {
-      this.mainTopicTitleVisible = false;
-    }
-  }
-
   /**
    * Called whenever a topic route is entered. Sets the current topicInfo,
    * and makes a guess about whether the main topic title is likely to be visible

spec/system/header_spec.rb

@@ -262,7 +262,7 @@ RSpec.describe "Glimmer Header", type: :system do
 
   describe "mobile topic-info" do
     fab!(:topic)
-    fab!(:posts) { Fabricate.times(5, :post, topic: topic) }
+    fab!(:posts) { Fabricate.times(21, :post, topic: topic) }
 
     it "only shows when scrolled down", mobile: true do
       visit "/t/#{topic.slug}/#{topic.id}"
@@ -286,5 +286,19 @@ RSpec.describe "Glimmer Header", type: :system do
       expect(page).not_to have_css("header.d-header .auth-buttons .login-button") # No header buttons
       expect(page).to have_css("header.d-header .title-wrapper .topic-link") # Title is shown in header
     end
+
+    it "shows when jumping from OP to much later post", mobile: true do
+      visit "/t/#{topic.slug}/#{topic.id}"
+
+      expect(page).to have_css("#topic-title") # Main topic title
+      expect(page).to have_css("header.d-header .auth-buttons .login-button") # header buttons visible when no topic-info in header
+
+      page.execute_script("Discourse.visit('/t/#{topic.slug}/#{topic.id}/21');")
+
+      expect(page).to have_css("#post_21")
+
+      expect(page).not_to have_css("header.d-header .auth-buttons .login-button") # No header buttons
+      expect(page).to have_css("header.d-header .title-wrapper .topic-link") # Title is shown in header
+    end
   end
 end