FIX: do not display the message content when it errors (#21008)

plugins/chat/spec/system/message_errors_spec.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+RSpec.describe "Message errors", type: :system, js: true do
+  fab!(:current_user) { Fabricate(:admin) }
+  let(:chat_page) { PageObjects::Pages::Chat.new }
+  let(:channel_page) { PageObjects::Pages::ChatChannel.new }
+  let(:max_length) { SiteSetting.chat_maximum_message_length }
+
+  before { chat_system_bootstrap }
+
+  context "when message is too long" do
+    fab!(:channel) { Fabricate(:chat_channel) }
+
+    it "only shows the error, not the message" do
+      channel.add(current_user)
+      sign_in(current_user)
+      chat_page.visit_channel(channel)
+
+      channel_page.send_message("atoolongmessage" + "a" * max_length)
+
+      expect(page).to have_no_content("atoolongmessage")
+      expect(page).to have_content(I18n.t("chat.errors.message_too_long", count: max_length))
+    end
+  end
+end