FIX: Sidebar custom sections rendering perf degrades over time (#21552)

What is the problem?

The main problem here is that we were incorrectly registering the same `onStateChange` callback with `TopicTrackingState`
each time a user reads a post. When a user reads a post, the state in `TopicTrackingState` is updated and it triggers all
the `onStateChange` callbacks which have been registered. In the `CommunitySection` class, we register a callback which
would then call the `onTopicTrackingStateChange` method for each link in the class. For the `EverythingSectionLink` class,
this would lookup the state in `TopicTrackingState` to get a new count of unread/new topics and update the `totalUnread` and
`totalNew` properties which are tracked. For some reason that I have yet to figure out, updating the either of the tracked properties
would result in Ember rerendering the entire `{{#each this.sections as |section|}}` in `component/sidebar/user/custom-sections.hbs`
template. Note that `this.sections` refers to a `@cached` getter in the `SidebarUserCustomSections` class. The problem is that
the `sections` getter is initializing a new bunch of sidebar sections related classes without calling the teardown function.
As a result, we end up registering new `onStateChange` callbacks in `TopicTrackingState` in `CommunitySection` without
removing the old ones. Over time, the number of callbacks build up and we end up slowing down the application. While we do
not know the reason why defining a getter for the `sections` is causing the entire block to re-render, I realized that
it is dangerous to use a getter for `sections` here since we have very little control on when the cached is broken.

Instead, I moved the `sections` getter to a tracked property instead where the property is updated via `appEvents`. With
this change, updating the tracked properties in `EverythingSectionLink` is no longer triggering a complete re-render of the
said block above. We also now call `teardown` on the section objects that has been initialised before updating the `sections`
property.

app/assets/javascripts/discourse/app/components/sidebar/user/custom-sections.js

@@ -2,9 +2,12 @@ import Component from "@glimmer/component";
 import { inject as service } from "@ember/service";
 import { ajax } from "discourse/lib/ajax";
 import { bind } from "discourse-common/utils/decorators";
-import { cached } from "@glimmer/tracking";
 import Section from "discourse/lib/sidebar/section";
 import CommunitySection from "discourse/lib/sidebar/community-section";
+import { tracked } from "@glimmer/tracking";
+
+export const REFRESH_CUSTOM_SIDEBAR_SECTIONS_APP_EVENT_NAME =
+  "sidebar:refresh-custom-sections";
 
 export default class SidebarUserCustomSections extends Component {
   @service currentUser;
@@ -15,21 +18,45 @@ export default class SidebarUserCustomSections extends Component {
   @service site;
   @service siteSettings;
 
+  @tracked sections = [];
+
   constructor() {
     super(...arguments);
     this.messageBus.subscribe("/refresh-sidebar-sections", this._refresh);
+
+    this.appEvents.on(
+      REFRESH_CUSTOM_SIDEBAR_SECTIONS_APP_EVENT_NAME,
+      this,
+      this._refreshSections
+    );
+
+    this.#initSections();
   }
 
   willDestroy() {
+    this.appEvents.off(
+      REFRESH_CUSTOM_SIDEBAR_SECTIONS_APP_EVENT_NAME,
+      this,
+      this._refreshSections
+    );
+
     this.messageBus.unsubscribe("/refresh-sidebar-sections");
+    this.#teardown();
+  }
+
+  #teardown() {
     return this.sections.forEach((section) => {
       section.teardown?.();
     });
   }
 
-  @cached
-  get sections() {
-    return this.currentUser.sidebarSections.map((section) => {
+  _refreshSections() {
+    this.#teardown();
+    this.#initSections();
+  }
+
+  #initSections() {
+    this.sections = this.currentUser.sidebarSections.map((section) => {
       switch (section.section_type) {
         case "community":
           const systemSection = new CommunitySection({
@@ -62,7 +89,7 @@ export default class SidebarUserCustomSections extends Component {
   @bind
   _refresh() {
     return ajax("/sidebar_sections.json", {}).then((json) => {
-      this.currentUser.set("sidebar_sections", json.sidebar_sections);
+      this.currentUser.updateSidebarSections(json.sidebar_sections);
     });
   }
 }

app/assets/javascripts/discourse/app/controllers/sidebar-section-form.js

@@ -268,8 +268,7 @@ export default Controller.extend(ModalFunctionality, {
       }),
     })
       .then((data) => {
-        this.currentUser.set(
-          "sidebar_sections",
+        this.currentUser.updateSidebarSections(
           this.currentUser.sidebar_sections.concat(data.sidebar_section)
         );
         this.send("closeModal");
@@ -310,7 +309,7 @@ export default Controller.extend(ModalFunctionality, {
             return section;
           }
         );
-        this.currentUser.set("sidebar_sections", newSidebarSections);
+        this.currentUser.updateSidebarSections(newSidebarSections);
         this.send("closeModal");
       })
       .catch((e) =>
@@ -360,7 +359,7 @@ export default Controller.extend(ModalFunctionality, {
                 this.currentUser.sidebar_sections.filter((section) => {
                   return section.id !== data["sidebar_section"].id;
                 });
-              this.currentUser.set("sidebar_sections", newSidebarSections);
+              this.currentUser.updateSidebarSections(newSidebarSections);
               this.send("closeModal");
             })
             .catch((e) =>

app/assets/javascripts/discourse/app/lib/sidebar/community-section.js

@@ -95,6 +95,7 @@ export default class CommunitySection {
     if (this.callbackId) {
       this.topicTrackingState.offStateChange(this.callbackId);
     }
+
     [...this.links, ...this.moreLinks].forEach((sectionLink) => {
       sectionLink.teardown?.();
     });

app/assets/javascripts/discourse/app/lib/sidebar/user/community-section/review-section-link.js

@@ -12,21 +12,18 @@ export default class ReviewSectionLink extends BaseSectionLink {
     super(...arguments);
 
     this._refreshCanDisplay();
-    if (this.shouldDisplay) {
-      this.appEvents.on(
-        "user-reviewable-count:changed",
-        this._refreshCanDisplay
-      );
-    }
+
+    this.appEvents?.on(
+      "user-reviewable-count:changed",
+      this._refreshCanDisplay
+    );
   }
 
   teardown() {
-    if (this.shouldDisplay) {
-      this.appEvents.off(
-        "user-reviewable-count:changed",
-        this._refreshCanDisplay
-      );
-    }
+    this.appEvents?.off(
+      "user-reviewable-count:changed",
+      this._refreshCanDisplay
+    );
   }
 
   @bind

app/assets/javascripts/discourse/app/models/user.js

@@ -51,6 +51,7 @@ import {
   showUserTip,
 } from "discourse/lib/user-tips";
 import { dependentKeyCompat } from "@ember/object/compat";
+import { REFRESH_CUSTOM_SIDEBAR_SECTIONS_APP_EVENT_NAME } from "discourse/components/sidebar/user/custom-sections";
 
 export const SECOND_FACTOR_METHODS = {
   TOTP: 1,
@@ -1161,6 +1162,11 @@ const User = RestModel.extend({
     this.appEvents.trigger("user-reviewable-count:changed", count);
   },
 
+  updateSidebarSections(sections) {
+    this.set("sidebar_sections", sections);
+    this.appEvents.trigger(REFRESH_CUSTOM_SIDEBAR_SECTIONS_APP_EVENT_NAME);
+  },
+
   isInDoNotDisturb() {
     return (
       this.do_not_disturb_until &&

app/assets/javascripts/discourse/tests/acceptance/sidebar-user-community-section-test.js

@@ -791,6 +791,14 @@ acceptance("Sidebar - Logged on user - Community Section", function (needs) {
       "shows suffix indicator for unread posts on everything link"
     );
 
+    const topicTrackingState = this.container.lookup(
+      "service:topic-tracking-state"
+    );
+
+    const initialCallbackCount = Object.keys(
+      topicTrackingState.stateChangeCallbacks
+    ).length;
+
     // simulate reading topic 2
     await publishToMessageBus("/unread", {
       topic_id: 2,
@@ -809,6 +817,12 @@ acceptance("Sidebar - Logged on user - Community Section", function (needs) {
       "shows suffix indicator for new topics on categories link"
     );
 
+    assert.equal(
+      Object.keys(topicTrackingState.stateChangeCallbacks).length,
+      initialCallbackCount,
+      "it does not add a new topic tracking state callback when the topic is read"
+    );
+
     // simulate reading topic 1
     await publishToMessageBus("/unread", {
       topic_id: 1,