Fixes diff view cookie being set on wrong endpoint

This fixes an issue only present with unified diff lines
where the diff view cookie would always be set as inline
because the cookie gets set on the batch endpoint

app/controllers/projects/merge_requests/diffs_controller.rb

@@ -4,7 +4,6 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
   include DiffHelper
   include RendersNotes
 
-  before_action :apply_diff_view_cookie!
   before_action :commit
   before_action :define_diff_vars
   before_action :define_diff_comment_vars, except: [:diffs_batch, :diffs_metadata]

app/controllers/projects/merge_requests_controller.rb

@@ -10,8 +10,10 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   include IssuableCollections
   include RecordUserLastActivity
   include SourcegraphDecorator
+  include DiffHelper
 
   skip_before_action :merge_request, only: [:index, :bulk_update]
+  before_action :apply_diff_view_cookie!, only: [:show]
   before_action :whitelist_query_limiting, only: [:assign_related_issues, :update]
   before_action :authorize_update_issuable!, only: [:close, :edit, :update, :remove_wip, :sort]
   before_action :authorize_read_actual_head_pipeline!, only: [

spec/controllers/projects/merge_requests/diffs_controller_spec.rb

@@ -58,39 +58,6 @@ RSpec.describe Projects::MergeRequests::DiffsController do
     end
   end
 
-  shared_examples 'persisted preferred diff view cookie' do
-    context 'with view param' do
-      before do
-        go(view: 'parallel')
-      end
-
-      it 'saves the preferred diff view in a cookie' do
-        expect(response.cookies['diff_view']).to eq('parallel')
-      end
-
-      it 'only renders the required view', :aggregate_failures do
-        diff_files_without_deletions = json_response['diff_files'].reject { |f| f['deleted_file'] }
-        have_no_inline_diff_lines = satisfy('have no inline diff lines') do |diff_file|
-          !diff_file.has_key?('highlighted_diff_lines')
-        end
-
-        expect(diff_files_without_deletions).to all(have_key('parallel_diff_lines'))
-        expect(diff_files_without_deletions).to all(have_no_inline_diff_lines)
-      end
-    end
-
-    context 'when the user cannot view the merge request' do
-      before do
-        project.team.truncate
-        go
-      end
-
-      it 'returns a 404' do
-        expect(response).to have_gitlab_http_status(:not_found)
-      end
-    end
-  end
-
   shared_examples "diff note on-demand position creation" do
     it "updates diff discussion positions" do
       service = double("service")
@@ -155,7 +122,6 @@ RSpec.describe Projects::MergeRequests::DiffsController do
       it_behaves_like 'forked project with submodules'
     end
 
-    it_behaves_like 'persisted preferred diff view cookie'
     it_behaves_like 'cached diff collection'
     it_behaves_like 'diff note on-demand position creation'
   end
@@ -511,7 +477,6 @@ RSpec.describe Projects::MergeRequests::DiffsController do
     end
 
     it_behaves_like 'forked project with submodules'
-    it_behaves_like 'persisted preferred diff view cookie'
     it_behaves_like 'cached diff collection'
 
     context 'diff unfolding' do

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -44,6 +44,16 @@ RSpec.describe Projects::MergeRequestsController do
       get :show, params: params.merge(extra_params)
     end
 
+    context 'with view param' do
+      before do
+        go(view: 'parallel')
+      end
+
+      it 'saves the preferred diff view in a cookie' do
+        expect(response.cookies['diff_view']).to eq('parallel')
+      end
+    end
+
     context 'when merge request is unchecked' do
       before do
         merge_request.mark_as_unchecked!