Fix missing resolve button when replying to notes

This clears the memoization of a note's discussion when we convert
it to a thread. We use this value when returning the JSON response
so it is important that it is accurate.

app/models/discussion.rb

@@ -23,6 +23,7 @@ class Discussion
             :resolved_by_id,
             :system_note_with_references_visible_for?,
             :resource_parent,
+            :save,
 
             to: :first_note
 

app/models/individual_note_discussion.rb

@@ -17,12 +17,8 @@ class IndividualNoteDiscussion < Discussion
     noteable.supports_replying_to_individual_notes?
   end
 
-  def convert_to_discussion!(save: false)
-    first_note.becomes!(Discussion.note_class).to_discussion.tap do
-      # Save needs to be called on first_note instead of the transformed note
-      # because of https://gitlab.com/gitlab-org/gitlab-foss/issues/57324
-      first_note.save if save
-    end
+  def convert_to_discussion!
+    first_note.becomes!(Discussion.note_class).to_discussion
   end
 
   def reply_attributes

app/services/notes/create_service.rb

@@ -54,7 +54,8 @@ module Notes
 
     def when_saved(note)
       if note.part_of_discussion? && note.discussion.can_convert_to_discussion?
-        note.discussion.convert_to_discussion!(save: true)
+        note.discussion.convert_to_discussion!.save
+        note.clear_memoization(:discussion)
       end
 
       todo_service.new_note(note, current_user)

changelogs/unreleased/233386-fix-resolve-button-when-replying-to-comment.yml

+---
+title: Fix missing resolve button when replying to notes in MRs
+merge_request: 39614
+author:
+type: fixed

spec/features/merge_request/user_posts_notes_spec.rb

@@ -95,20 +95,32 @@ RSpec.describe 'Merge request > User posts notes', :js do
     end
   end
 
-  describe 'reply on a deleted conversation' do
-    before do
-      visit project_merge_request_path(project, merge_request)
-    end
-
-    it 'shows an error message' do
+  describe 'replying to a comment' do
+    it 'makes the discussion resolvable' do
       find('.js-reply-button').click
-      note.delete
 
       page.within('.discussion-reply-holder') do
         fill_in 'note[note]', with: 'A reply'
         click_button 'Add comment now'
-        wait_for_requests
-        expect(page).to have_content('Your comment could not be submitted because discussion to reply to cannot be found')
+
+        expect(page).to have_button('Resolve thread')
+      end
+    end
+
+    context 'when comment is deleted' do
+      before do
+        note.delete
+      end
+
+      it 'shows an error message' do
+        find('.js-reply-button').click
+
+        page.within('.discussion-reply-holder') do
+          fill_in 'note[note]', with: 'A reply'
+          click_button 'Add comment now'
+
+          expect(page).to have_content('Your comment could not be submitted because discussion to reply to cannot be found')
+        end
       end
     end
   end

spec/services/notes/create_service_spec.rb

@@ -434,6 +434,13 @@ RSpec.describe Notes::CreateService do
             .and change { existing_note.updated_at }
       end
 
+      it 'returns a DiscussionNote with its parent discussion refreshed correctly' do
+        discussion_notes = subject.discussion.notes
+
+        expect(discussion_notes.size).to eq(2)
+        expect(discussion_notes.first).to be_a(DiscussionNote)
+      end
+
       context 'discussion to reply cannot be found' do
         before do
           existing_note.delete