Add end-to-end test for issue mention

qa/qa/specs/features/browser_ui/2_plan/issue/mentions_spec.rb

+# frozen_string_literal: true
+
+module QA
+  context 'Plan' do
+    describe 'mention' do
+      let(:user) do
+        Resource::User.fabricate_via_api! do |user|
+          user.name = "bob"
+          user.password = "1234test"
+        end
+      end
+
+      before do
+        QA::Runtime::Env.personal_access_token = QA::Runtime::Env.admin_personal_access_token
+
+        unless QA::Runtime::Env.personal_access_token
+          Runtime::Browser.visit(:gitlab, Page::Main::Login)
+          Page::Main::Login.perform(&:sign_in_using_admin_credentials)
+        end
+
+        QA::Runtime::Env.personal_access_token = nil
+
+        Page::Main::Menu.perform(&:sign_out) if Page::Main::Menu.perform { |p| p.has_personal_area?(wait: 0) }
+
+        Runtime::Browser.visit(:gitlab, Page::Main::Login)
+
+        Page::Main::Login.perform(&:sign_in_using_credentials)
+
+        project = Resource::Project.fabricate_via_api! do |resource|
+          resource.name = 'project-to-test-mention'
+        end
+        project.visit!
+
+        Page::Project::Show.perform(&:go_to_members_settings)
+        Page::Project::Settings::Members.perform do |members|
+          members.add_member(user.username)
+        end
+
+        issue = Resource::Issue.fabricate_via_api! do |issue|
+          issue.title = 'issue to test mention'
+          issue.project = project
+        end
+        issue.visit!
+      end
+
+      it 'user mentions another user in an issue' do
+        Page::Project::Issue::Show.perform do |show|
+          at_username = "@#{user.username}"
+
+          show.select_all_activities_filter
+          show.comment(at_username)
+
+          expect(show).to have_link(at_username)
+        end
+      end
+    end
+  end
+end