Show user popovers on system notes

This way we are consistently initializing user popovers using
the js-user-link class, rather than a mix of that and the gfm
class. This might potentially have terrible knock-on effects
but seems to be mostly ok

app/assets/javascripts/behaviors/markdown/render_gfm.js

@@ -16,7 +16,7 @@ $.fn.renderGFM = function renderGFM() {
   renderMath(this.find('.js-render-math'));
   renderMermaid(this.find('.js-render-mermaid'));
   highlightCurrentUser(this.find('.gfm-project_member').get());
-  initUserPopovers(this.find('.gfm-project_member').get());
+  initUserPopovers(this.find('.js-user-link').get());
   initMRPopovers(this.find('.gfm-merge_request').get());
   renderMetrics(this.find('.js-render-metrics').get());
   return this;

changelogs/unreleased/38355-user-popovers-don-t-work-in-system-notes.yml

+---
+title: Add user popovers to system notes
+merge_request: 24241
+author:
+type: fixed

lib/banzai/filter/user_reference_filter.rb

@@ -106,7 +106,7 @@ module Banzai
       end
 
       def link_class
-        reference_class(:project_member, tooltip: false)
+        [reference_class(:project_member, tooltip: false), "js-user-link"].join(" ")
       end
 
       def link_to_all(link_content: nil)

spec/features/projects/user_sees_user_popover_spec.rb

@@ -3,6 +3,8 @@
 require 'spec_helper'
 
 describe 'User sees user popover', :js do
+  include Spec::Support::Helpers::Features::NotesHelpers
+
   let_it_be(:project) { create(:project, :repository) }
   let(:user) { project.creator }
   let(:merge_request) do
@@ -17,13 +19,13 @@ describe 'User sees user popover', :js do
   subject { page }
 
   describe 'hovering over a user link in a merge request' do
+    let(:popover_selector) { '.user-popover' }
+
     before do
       visit project_merge_request_path(project, merge_request)
     end
 
     it 'displays user popover' do
-      popover_selector = '.user-popover'
-
       find('.js-user-link').hover
 
       expect(page).to have_css(popover_selector, visible: true)
@@ -32,5 +34,17 @@ describe 'User sees user popover', :js do
         expect(page).to have_content(user.name)
       end
     end
+
+    it "displays user popover in system note" do
+      add_note("/assign @#{user.username}")
+
+      wait_for_requests
+
+      find('.system-note-message .js-user-link').hover
+
+      page.within(popover_selector) do
+        expect(page).to have_content(user.name)
+      end
+    end
   end
 end

spec/lib/banzai/filter/user_reference_filter_spec.rb

@@ -123,7 +123,7 @@ describe Banzai::Filter::UserReferenceFilter do
 
   it 'includes default classes' do
     doc = reference_filter("Hey #{reference}")
-    expect(doc.css('a').first.attr('class')).to eq 'gfm gfm-project_member'
+    expect(doc.css('a').first.attr('class')).to eq 'gfm gfm-project_member js-user-link'
   end
 
   context 'when a project is not specified' do