Fix error with loading outdated diff lines

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/346758

app/controllers/projects/notes_controller.rb

@@ -57,7 +57,7 @@ class Projects::NotesController < Projects::ApplicationController
 
   def outdated_line_change
     diff_lines = Rails.cache.fetch(['note', note.id, 'oudated_line_change'], expires_in: 7.days) do
-      ::MergeRequests::OutdatedDiscussionDiffLinesService.new(project: @project, note: note).execute.to_json
+      ::MergeRequests::OutdatedDiscussionDiffLinesService.new(project: note.noteable.source_project, note: note).execute.to_json
     end
 
     render json: diff_lines

app/models/note.rb

@@ -603,6 +603,15 @@ class Note < ApplicationRecord
     })
   end
 
+  def show_outdated_changes?
+    return false unless for_merge_request?
+    return false unless Feature.enabled?(:display_outdated_line_diff, noteable.source_project, default_enabled: :yaml)
+    return false unless system?
+    return false unless change_position&.line_range
+
+    change_position.line_range["end"] || change_position.line_range["start"]
+  end
+
   private
 
   def system_note_viewable_by?(user)

app/serializers/note_entity.rb

@@ -51,7 +51,7 @@ class NoteEntity < API::Entities::Note
     SystemNoteHelper.system_note_icon_name(note)
   end
 
-  expose :outdated_line_change_path, if: -> (note, _) { note.system? && note.change_position&.line_range && Feature.enabled?(:display_outdated_line_diff, note.project, default_enabled: :yaml) } do |note|
+  expose :outdated_line_change_path, if: -> (note, _) { note.show_outdated_changes? } do |note|
     outdated_line_change_namespace_project_note_path(namespace_id: note.project.namespace, project_id: note.project, id: note)
   end
 

app/services/merge_requests/outdated_discussion_diff_lines_service.rb

@@ -14,12 +14,12 @@ module MergeRequests
     end
 
     def execute
-      end_position = position.line_range["end"]
+      line_position = position.line_range["end"] || position.line_range["start"]
       diff_line_index = diff_lines.find_index do |l|
-        if end_position["new_line"]
-          l.new_line == end_position["new_line"]
-        elsif end_position["old_line"]
-          l.old_line == end_position["old_line"]
+        if line_position["new_line"]
+          l.new_line == line_position["new_line"]
+        elsif line_position["old_line"]
+          l.old_line == line_position["old_line"]
         end
       end
       initial_line_index = [diff_line_index - OVERFLOW_LINES_COUNT, 0].max