Fix bug in personal snippet thread discussions

In personal snippets, there is a bug when starting a
thread, the visual component to response is not displayed.

This is because the object `@snippet` is not set in the
controller and we end up in a policy check without object.

app/controllers/snippets/notes_controller.rb

@@ -23,7 +23,7 @@ class Snippets::NotesController < ApplicationController
 
   # rubocop: disable CodeReuse/ActiveRecord
   def snippet
-    PersonalSnippet.find_by(id: params[:snippet_id])
+    @snippet ||= PersonalSnippet.find_by(id: params[:snippet_id])
   end
   # rubocop: enable CodeReuse/ActiveRecord
   alias_method :noteable, :snippet

changelogs/unreleased/299719-fj-fix-bug-personal-snippet-thread-discussion.yml

+---
+title: Fix bug in personal snippet thread discussions
+merge_request: 52490
+author:
+type: fixed

spec/features/discussion_comments/snippets_spec.rb

@@ -4,15 +4,34 @@ require 'spec_helper'
 
 RSpec.describe 'Thread Comments Snippet', :js do
   let_it_be(:user) { create(:user) }
-  let_it_be(:project) { create(:project) }
-  let_it_be(:snippet) { create(:project_snippet, :private, :repository, project: project, author: user) }
 
   before do
-    project.add_maintainer(user)
     sign_in(user)
+  end
+
+  context 'with project snippets' do
+    let_it_be(:project) do
+      create(:project).tap do |p|
+        p.add_maintainer(user)
+      end
+    end
+
+    let_it_be(:snippet) { create(:project_snippet, :private, :repository, project: project, author: user) }
+
+    before do
+      visit project_snippet_path(project, snippet)
+    end
 
-    visit project_snippet_path(project, snippet)
+    it_behaves_like 'thread comments', 'snippet'
   end
 
-  it_behaves_like 'thread comments', 'snippet'
+  context 'with personal snippets' do
+    let_it_be(:snippet) { create(:personal_snippet, :private, :repository, author: user) }
+
+    before do
+      visit snippet_path(snippet)
+    end
+
+    it_behaves_like 'thread comments', 'snippet'
+  end
 end

spec/support/shared_examples/features/discussion_comments_shared_example.rb

@@ -150,12 +150,13 @@ RSpec.shared_examples 'thread comments' do |resource_name|
           wait_for_requests
         end
 
-        it 'clicking "Start thread" will post a thread' do
+        it 'clicking "Start thread" will post a thread and show a reply component' do
           expect(page).to have_content(comment)
 
           new_comment = all(comments_selector).last
 
           expect(new_comment).to have_selector('.discussion')
+          expect(new_comment).to have_css('.discussion-with-resolve-btn')
         end
 
         if resource_name =~ /(issue|merge request)/