FIX: ensures chat sidebar is present when core sidebar is disabled (#19197)

plugins/chat/assets/javascripts/discourse/controllers/chat.js

@@ -1,12 +1,25 @@
 import Controller from "@ember/controller";
-import { action } from "@ember/object";
 import { inject as service } from "@ember/service";
 
 export default class ChatController extends Controller {
   @service chat;
 
-  @action
-  switchChannel(channel) {
-    this.chat.openChannel(channel);
+  get shouldUseChatSidebar() {
+    if (this.site.mobileView) {
+      return false;
+    }
+
+    if (this.shouldUseCoreSidebar) {
+      return false;
+    }
+
+    return true;
+  }
+
+  get shouldUseCoreSidebar() {
+    return (
+      this.siteSettings.enable_sidebar &&
+      this.siteSettings.enable_experimental_sidebar_hamburger
+    );
   }
 }

plugins/chat/assets/javascripts/discourse/templates/chat.hbs

@@ -11,15 +11,13 @@
     class={{concat-class
       "full-page-chat"
       (if
-        this.siteSettings.enable_sidebar
+        this.shouldUseCoreSidebar
         "full-page-chat-sidebar-enabled"
       )
     }}
   >
-    {{#if
-      (and (not this.siteSettings.enable_sidebar) (not this.site.mobileView))
-    }}
-      <ChannelsList @onSelect={{action "switchChannel"}} />
+    {{#if this.shouldUseChatSidebar}}
+      <ChannelsList />
     {{/if}}
 
     <div id="main-chat-outlet">

plugins/chat/spec/system/sidebars_spec.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+RSpec.describe "Navigation", type: :system, js: true do
+  fab!(:category) { Fabricate(:category) }
+  fab!(:topic) { Fabricate(:topic) }
+  fab!(:post) { Fabricate(:post, topic: topic) }
+  fab!(:user) { Fabricate(:admin) }
+  fab!(:category_channel) { Fabricate(:category_channel) }
+  fab!(:category_channel_2) { Fabricate(:category_channel) }
+  let(:chat_page) { PageObjects::Pages::Chat.new }
+
+  before do
+    chat_system_bootstrap(user, [category_channel, category_channel_2])
+    sign_in(user)
+  end
+
+  context "when core sidebar is enabled" do
+    before do
+      SiteSetting.enable_sidebar = true
+      SiteSetting.enable_experimental_sidebar_hamburger = true
+    end
+
+    it "uses core sidebar" do
+      visit("/chat")
+
+      expect(page).to have_css("#d-sidebar")
+      expect(page).to_not have_css(".channels-list")
+    end
+
+    context "when visiting on mobile" do
+      it "has no sidebar" do
+        visit("/?mobile_view=1")
+        chat_page.visit_channel(category_channel_2)
+
+        expect(page).to_not have_css("#d-sidebar")
+      end
+    end
+  end
+
+  it "uses chat sidebar" do
+    visit("/chat")
+
+    expect(page).to have_css(".channels-list")
+    expect(page).to_not have_css("#d-sidebar")
+  end
+
+  context "when visiting on mobile" do
+    it "has no sidebar" do
+      visit("/?mobile_view=1")
+      chat_page.visit_channel(category_channel_2)
+
+      expect(page).to_not have_css(".channels-list")
+    end
+  end
+end