FIX: new message brings a direct message channel to the top (#19332)

Co-authored-by: Alan Guo Xiang Tan <gxtan1990@gmail.com>

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

@@ -750,9 +750,13 @@ export default class Chat extends Service {
         this.userChatChannelTrackingStateChanged();
         channel.set("last_message_sent_at", new Date());
 
-        (this.directMessageChannels || [])
-          .findBy("id", parseInt(channel.id, 10))
-          ?.reSortDirectMessageChannels?.();
+        const directMessageChannel = (this.directMessageChannels || []).findBy(
+          "id",
+          parseInt(channel.id, 10)
+        );
+        if (directMessageChannel) {
+          this.reSortDirectMessageChannels();
+        }
       },
       channel.message_bus_last_ids.new_messages
     );

plugins/chat/spec/system/receiving_message_spec.rb

@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+RSpec.describe "Receiving message", type: :system, js: true do
+  fab!(:current_user) { Fabricate(:admin) }
+  fab!(:user_2) { Fabricate(:user) }
+  fab!(:user_3) { Fabricate(:user) }
+  fab!(:direct_message_channel_1) do
+    Fabricate(:direct_message_channel, users: [current_user, user_2])
+  end
+  fab!(:direct_message_channel_2) do
+    Fabricate(:direct_message_channel, users: [current_user, user_2, user_3])
+  end
+
+  let(:chat_page) { PageObjects::Pages::Chat.new }
+
+  before do
+    chat_system_bootstrap(current_user, [direct_message_channel_1, direct_message_channel_2])
+    sign_in(current_user)
+  end
+
+  context "when core sidebar is enabled" do
+    before do
+      SiteSetting.enable_sidebar = true
+      SiteSetting.enable_experimental_sidebar_hamburger = true
+    end
+
+    context "when receiving a direct message" do
+      it "reorders the direct messages sidebar section links ordering by last message received" do
+        visit("/")
+
+        expect(
+          page.find(
+            "#sidebar-section-content-chat-dms .sidebar-section-link-wrapper:nth-child(2) .sidebar-row",
+          ),
+        ).to have_content(direct_message_channel_2.name)
+
+        creator =
+          Chat::ChatMessageCreator.create(
+            chat_channel: direct_message_channel_2,
+            user: user_2,
+            content: "this is good",
+          )
+
+        expect(
+          page.find(
+            "#sidebar-section-content-chat-dms .sidebar-section-link-wrapper:nth-child(1) .sidebar-row",
+          ),
+        ).to have_content(direct_message_channel_2.name)
+      end
+    end
+  end
+end