Fix flaky file locking spec

Added wait for merge request error message

app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_failed_to_merge.vue

@@ -84,7 +84,12 @@ export default {
           <span v-else> {{ s__('mrWidget|Merge failed.') }} </span>
           <span :class="{ 'has-custom-error': mr.mergeError }"> {{ timerText }} </span>
         </span>
-        <button class="btn btn-default btn-sm js-refresh-button" type="button" @click="refresh">
+        <button
+          class="btn btn-default btn-sm js-refresh-button"
+          data-qa-selector="merge_request_error_content"
+          type="button"
+          @click="refresh"
+        >
           {{ s__('mrWidget|Refresh now') }}
         </button>
       </div>

qa/qa/page/merge_request/show.rb

@@ -29,6 +29,10 @@ module QA
           element :merged_status_content
         end
 
+        view 'app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_failed_to_merge.vue' do
+          element :merge_request_error_content
+        end
+
         view 'app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_rebase.vue' do
           element :mr_rebase_button
           element :no_fast_forward_message, 'Fast-forward merge is not possible' # rubocop:disable QA/ElementWithPattern
@@ -193,6 +197,12 @@ module QA
           click_element :dropdown_toggle
           visit_link_in_element(:download_plain_diff)
         end
+
+        def wait_for_merge_request_error_message
+          wait(max: 30, reload: false) do
+            has_element?(:merge_request_error_content)
+          end
+        end
       end
     end
   end

qa/qa/specs/features/ee/browser_ui/3_create/repository/file_locking_spec.rb

@@ -59,9 +59,8 @@ module QA
         end
       end
 
-      # https://gitlab.com/gitlab-org/gitlab/issues/40125
-      # BUG_IN_CODE
-      it 'creates a merge request and fails to merge', :quarantine do
+      # Known issue: https://gitlab.com/gitlab-org/gitlab/issues/40125
+      it 'creates a merge request and fails to merge' do
         push branch: 'test', as_user: @user_one
 
         merge_request = Resource::MergeRequest.fabricate_via_api! do |merge_request|
@@ -75,6 +74,7 @@ module QA
         click_lock
         sign_out_and_sign_in_as user: @user_one
         try_to_merge merge_request: merge_request
+        Page::MergeRequest::Show.perform(&:wait_for_merge_request_error_message)
         expect(page).to have_text("locked by #{admin_username}")
       end