Add feature flag to remove startup CSS

app/helpers/application_helper.rb

@@ -281,7 +281,7 @@ def stylesheet_link_tag_defer(path)
   end
 
   def startup_css_enabled?
-    !params.has_key?(:no_startup_css)
+    !Feature.enabled?(:remove_startup_css) && !params.has_key?(:no_startup_css)
   end
 
   def sign_in_with_redirect?

config/feature_flags/development/remove_startup_css.yml

+---
+name: remove_startup_css
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/117495
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/407140
+milestone: '16.0'
+type: development
+group: group::project management
+default_enabled: false

spec/helpers/application_helper_spec.rb

@@ -696,10 +696,18 @@ def stub_controller_method(method_name, value)
   end
 
   describe 'stylesheet_link_tag_defer' do
-    it 'uses print stylesheet by default' do
+    it 'uses print stylesheet when feature flag disabled' do
+      stub_feature_flags(remove_startup_css: false)
+
       expect(helper.stylesheet_link_tag_defer('test')).to eq( '<link rel="stylesheet" media="print" href="/stylesheets/test.css" />')
     end
 
+    it 'uses regular stylesheet when feature flag disabled' do
+      stub_feature_flags(remove_startup_css: true)
+
+      expect(helper.stylesheet_link_tag_defer('test')).to eq( '<link rel="stylesheet" media="screen" href="/stylesheets/test.css" />')
+    end
+
     it 'uses regular stylesheet when no_startup_css param present' do
       allow(helper.controller).to receive(:params).and_return({ no_startup_css: '' })
 

spec/views/layouts/_head.html.haml_spec.rb

@@ -59,7 +59,7 @@
 
     render
 
-    expect(rendered).to match('<link rel="stylesheet" media="print" href="/stylesheets/highlight/themes/solarised-light.css" />')
+    expect(rendered).to match('<link rel="stylesheet" media="screen" href="/stylesheets/highlight/themes/solarised-light.css" />')
   end
 
   context 'when an asset_host is set and snowplow url is set', quarantine: 'https://gitlab.com/gitlab-org/gitlab/-/issues/346542' do