FIX: Do not duplicate admin sidebar plugin links (#25780)

When hiding/showing the sidebar, as is the case on mobile
and using the toggle in the top left on desktop, we delete
and recreate the ember component on the page. This causes
the `sections` for each sidebar panel to get re-evaluated
every time.

For the admin sidebar, this means that we were constantly
re-adding the plugin links to the sidebar, causing duplication.
This can be fixed by just adding @cached to the getter for
sections.

app/assets/javascripts/discourse/app/lib/sidebar/admin-sidebar.js

@@ -1,3 +1,4 @@
+import { cached } from "@glimmer/tracking";
 import PreloadStore from "discourse/lib/preload-store";
 import { ADMIN_NAV_MAP } from "discourse/lib/sidebar/admin-nav-map";
 import BaseCustomSidebarPanel from "discourse/lib/sidebar/base-custom-sidebar-panel";
@@ -213,6 +214,7 @@ export default class AdminSidebarPanel extends BaseCustomSidebarPanel {
   key = ADMIN_PANEL;
   hidden = true;
 
+  @cached
   get sections() {
     const currentUser = getOwnerWithFallback().lookup("service:current-user");
     const siteSettings = getOwnerWithFallback().lookup("service:site-settings");

plugins/chat/spec/system/admin_sidebar_navigation_spec.rb

@@ -14,4 +14,12 @@ describe "Admin Revamp | Sidebar Navigation | Plugin Links", type: :system do
     visit("/admin")
     expect(sidebar).to have_section_link("Chat", href: "/admin/plugins/chat")
   end
+
+  it "does not duplicate links to enabled plugin admin routes when showing and hiding sidebar" do
+    visit("/admin")
+    expect(sidebar).to have_section_link("Chat", href: "/admin/plugins/chat", count: 1)
+    find(".header-sidebar-toggle").click
+    find(".header-sidebar-toggle").click
+    expect(sidebar).to have_section_link("Chat", href: "/admin/plugins/chat", count: 1)
+  end
 end

spec/system/page_objects/components/navigation_menu/base.rb

@@ -36,8 +36,15 @@ module PageObjects
           has_css?(".#{SIDEBAR_SECTION_LINK_SELECTOR}--active", count: 1)
         end
 
-        def has_section_link?(name, href: nil, active: false, target: nil)
+        def has_section_link?(name, href: nil, active: false, target: nil, count: 1)
-          section_link_present?(name, href: href, active: active, target: target, present: true)
+          section_link_present?(
+            name,
+            href: href,
+            active: active,
+            target: target,
+            present: true,
+            count: count,
+          )
         end
 
         def has_no_section_link?(name, href: nil, active: false)
@@ -163,12 +170,13 @@ module PageObjects
 
         private
 
-        def section_link_present?(name, href: nil, active: false, target: nil, present:)
+        def section_link_present?(name, href: nil, active: false, target: nil, count: 1, present:)
           attributes = { exact_text: name }
           attributes[:href] = href if href
           attributes[:class] = SIDEBAR_SECTION_LINK_SELECTOR
           attributes[:class] += "--active" if active
           attributes[:target] = target if target
+          attributes[:count] = count
           page.public_send(present ? :has_link? : :has_no_link?, **attributes)
         end