Moving StartupCSS onload event handlers

Moved to inline script tag instead of inline event handlers

app/helpers/application_helper.rb

@@ -237,7 +237,7 @@ module ApplicationHelper
 
   def stylesheet_link_tag_defer(path)
     if use_startup_css?
-      stylesheet_link_tag(path, media: "print", onload: "this.onload=null;this.media='all'")
+      stylesheet_link_tag(path, media: "print")
     else
       stylesheet_link_tag(path, media: "all")
     end

app/views/layouts/_head.html.haml

@@ -57,10 +57,12 @@
   - else
     = stylesheet_link_tag_defer "application"
   = stylesheet_link_tag "disable_animations", media: "all" if Rails.env.test? || Gitlab.config.gitlab['disable_animations']
-  = stylesheet_link_tag 'performance_bar' if performance_bar_enabled?
+  = stylesheet_link_tag_defer 'performance_bar' if performance_bar_enabled?
 
   = stylesheet_link_tag_defer "highlight/themes/#{user_color_scheme}"
 
+  = render 'layouts/startup_css'
+
   = Gon::Base.render_data(nonce: content_security_policy_nonce)
 
   - if content_for?(:library_javascripts)

app/views/layouts/_startup_css.haml

+- return unless use_startup_css?
+
+= javascript_tag nonce: true do
+  :plain
+    document.querySelectorAll('link[media="print"]').forEach(linkTag => {
+      linkTag.addEventListener('load', function() {this.media='all'}, {once: true});
+    })