test: add new system test for --headless=old (#27834)

system-tests/projects/e2e/cypress/e2e/headless_old.cy.js

@@ -0,0 +1,5 @@
+describe('e2e headless old spec', function () {
+  it('has expected launch args', function () {
+    cy.task('get:browser:args').should('contain', '--headless=old')
+  })
+})

system-tests/projects/e2e/cypress/plugins/index.js

@@ -59,6 +59,17 @@ module.exports = (on, config) => {
     }
 
     if (browser.family === 'chromium' && browser.name !== 'electron') {
+      if (process.env.CHROMIUM_USE_HEADLESS_OLD) {
+        options.args = options.args.map((arg) => {
+          // ensure we are using --headless=old by overriding both headless new and default
+          if (arg === '--headless' || arg === '--headless=new') {
+            return '--headless=old'
+          }
+
+          return arg
+        })
+      }
+
       if (process.env.CHROMIUM_EXTRA_LAUNCH_ARGS) {
         options.args = options.args.concat(process.env.CHROMIUM_EXTRA_LAUNCH_ARGS.split(' '))
       }

system-tests/test/headless_old_spec.js

@@ -0,0 +1,13 @@
+const systemTests = require('../lib/system-tests').default
+
+describe('e2e', () => {
+  systemTests.setup()
+
+  systemTests.it('succeeds using --headless=old', {
+    spec: 'headless_old.cy.js',
+    browser: 'chrome',
+    processEnv: {
+      CHROMIUM_USE_HEADLESS_OLD: 1,
+    },
+  })
+})