Display notes on merge ref head diff

It enables merge_ref_head_comments FF by default

app/controllers/projects/merge_requests_controller.rb

@@ -31,7 +31,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     push_frontend_feature_flag(:suggest_pipeline) if experiment_enabled?(:suggest_pipeline)
     push_frontend_feature_flag(:code_navigation, @project, default_enabled: true)
     push_frontend_feature_flag(:widget_visibility_polling, @project, default_enabled: true)
-    push_frontend_feature_flag(:merge_ref_head_comments, @project)
+    push_frontend_feature_flag(:merge_ref_head_comments, @project, default_enabled: true)
     push_frontend_feature_flag(:mr_commit_neighbor_nav, @project, default_enabled: true)
     push_frontend_feature_flag(:multiline_comments, @project)
     push_frontend_feature_flag(:file_identifier_hash)

app/serializers/discussion_entity.rb

@@ -72,6 +72,6 @@ class DiscussionEntity < Grape::Entity
     return unless discussion.diff_discussion?
     return if discussion.legacy_diff_discussion?
 
-    Feature.enabled?(:merge_ref_head_comments, discussion.project)
+    Feature.enabled?(:merge_ref_head_comments, discussion.project, default_enabled: true)
   end
 end

app/services/merge_requests/mergeability_check_service.rb

@@ -125,7 +125,7 @@ module MergeRequests
     end
 
     def update_diff_discussion_positions!
-      return if Feature.disabled?(:merge_ref_head_comments, merge_request.target_project)
+      return if Feature.disabled?(:merge_ref_head_comments, merge_request.target_project, default_enabled: true)
 
       Discussions::CaptureDiffNotePositionsService.new(merge_request).execute
     end

app/services/notes/create_service.rb

@@ -66,7 +66,7 @@ module Notes
         Gitlab::Tracking.event('Notes::CreateService', 'execute', tracking_data_for(note))
       end
 
-      if Feature.enabled?(:merge_ref_head_comments, project) && note.for_merge_request? && note.diff_note? && note.start_of_discussion?
+      if Feature.enabled?(:merge_ref_head_comments, project, default_enabled: true) && note.for_merge_request? && note.diff_note? && note.start_of_discussion?
         Discussions::CaptureDiffNotePositionService.new(note.noteable, note.diff_file&.paths).execute(note.discussion)
       end
     end

changelogs/unreleased/id-merge-ref-comments-ff.yml

+---
+title: Display notes on merge ref head diff
+merge_request: 35422
+author:
+type: added