Issue #3344127: Composer validate all fixture manipulation changes

package_manager/tests/src/Kernel/ComposerPluginsValidatorTest.php

@@ -237,6 +237,25 @@ class ComposerPluginsValidatorTest extends PackageManagerKernelTestBase {
       ],
       [],
     ];
+
+    yield 'one UNsupported but disallowed plugin' => [
+      [
+        'allow-plugins' => [
+          // Definitely NOT `composer/plugin-c`!
+          'drupal/core-project-message' => TRUE,
+        ],
+      ],
+      [
+        [
+          'name' => 'composer/plugin-c',
+          'version' => '16.4',
+          'type' => 'composer-plugin',
+          'require' => ['composer-plugin-api' => '*'],
+          'extra' => ['class' => 'AnyClass'],
+        ],
+      ],
+      [],
+    ];
   }
 
   public function providerSimpleInvalidCases(): \Generator {