FIX: deep linking to a message in a thread

Whenever you get a bookmark notification, a mention notification, or click on a bookmark on a message in a long thread, we should ensure we always highlight and show the proper message.

Before this fix, we would correctly load the thread, but would always start at the bottom.

Internal ref. t/128103

plugins/chat/assets/javascripts/discourse/components/chat-thread.gjs

@@ -223,11 +223,12 @@ export default class ChatThread extends Component {
 
     this.messagesManager.clear();
 
-    findArgs.targetMessageId ??=
+    findArgs.target_message_id ??=
+      findArgs.targetMessageId ||
       this.args.targetMessageId ||
       this.args.thread.currentUserMembership?.lastReadMessageId;
 
-    if (!findArgs.targetMessageId) {
+    if (!findArgs.target_message_id) {
       findArgs.direction = FUTURE;
     }
 

plugins/chat/spec/system/bookmark_message_spec.rb

@@ -5,6 +5,7 @@ RSpec.describe "Bookmark message", type: :system do
 
   let(:chat_page) { PageObjects::Pages::Chat.new }
   let(:channel_page) { PageObjects::Pages::ChatChannel.new }
+  let(:thread_page) { PageObjects::Pages::ChatThread.new }
   let(:bookmark_modal) { PageObjects::Modals::Bookmark.new }
 
   fab!(:category_channel_1) { Fabricate(:category_channel) }
@@ -27,6 +28,26 @@ RSpec.describe "Bookmark message", type: :system do
       expect(channel_page).to have_bookmarked_message(message_1)
     end
 
+    it "supports linking to a bookmark in a long thread" do
+      category_channel_1.update!(threading_enabled: true)
+      category_channel_1.add(current_user)
+
+      thread =
+        chat_thread_chain_bootstrap(
+          channel: category_channel_1,
+          users: [current_user, Fabricate(:user)],
+          messages_count: Chat::MessagesQuery::MAX_PAGE_SIZE + 1,
+        )
+
+      first_message = thread.replies.first
+
+      bookmark = Bookmark.create!(bookmarkable: first_message, user: current_user)
+
+      visit bookmark.bookmarkable.url
+
+      expect(thread_page).to have_bookmarked_message(first_message)
+    end
+
     context "when the user has a bookmark auto_delete_preference" do
       before do
         current_user.user_option.update!(

plugins/chat/spec/system/page_objects/chat/chat_thread.rb

@@ -140,6 +140,10 @@ module PageObjects
         messages.edit(message)
         send_message(message.message + " " + text) if text
       end
+
+      def has_bookmarked_message?(message)
+        find(message_by_id_selector(message.id) + ".-bookmarked")
+      end
     end
   end
 end