Add feature spec for real-time sidebar assignees

Also mounts ActionCable in Capybara's Puma server

config/initializers/action_cable.rb

 # frozen_string_literal: true
 
 Rails.application.configure do
-  # Prevents the default engine from being mounted because
-  # we're running ActionCable as a standalone server
-  config.action_cable.mount_path = nil
+  # We only mount the ActionCable engine in tests where we run it in-app
+  # For other environments, we run it on a standalone Puma server
+  config.action_cable.mount_path = Rails.env.test? ? '/-/cable' : nil
   config.action_cable.url = Gitlab::Utils.append_path(Gitlab.config.gitlab.relative_url_root, '/-/cable')
   config.action_cable.worker_pool_size = Gitlab.config.action_cable.worker_pool_size
 end

spec/features/issues/user_sees_sidebar_updates_in_realtime_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'Issues > Real-time sidebar', :js do
+  let_it_be(:project) { create(:project, :public) }
+  let_it_be(:issue) { create(:issue, project: project) }
+  let_it_be(:user) { create(:user) }
+
+  before_all do
+    project.add_developer(user)
+  end
+
+  it 'updates the assignee in real-time' do
+    Capybara::Session.new(:other_session)
+
+    using_session :other_session do
+      visit project_issue_path(project, issue)
+      expect(page.find('.assignee')).to have_content 'None'
+    end
+
+    gitlab_sign_in(user)
+    visit project_issue_path(project, issue)
+    expect(page.find('.assignee')).to have_content 'None'
+
+    click_button 'assign yourself'
+
+    using_session :other_session do
+      expect(page.find('.assignee')).to have_content user.name
+    end
+  end
+end