[BUGFIX] Drop only bootstrap-opened output buffers

Classes/Core/Functional/FunctionalTestCase.php

@@ -1005,6 +1005,7 @@ private function retrieveFrontendSubRequestResult(
         $_SERVER['SCRIPT_NAME'] = '/index.php';
         $_SERVER['HTTP_HOST'] = $_SERVER['SERVER_NAME'] = $request->getUri()->getHost() ?: 'localhost';
 
+        $outputBufferingLevel = ob_get_level();
         $container = Bootstrap::init(ClassLoadingInformation::getClassLoader());
 
         /** @var InternalRequest $serverRequest */
@@ -1050,8 +1051,12 @@ private function retrieveFrontendSubRequestResult(
             $frontendApplication = $container->get(Application::class);
             $response = $frontendApplication->handle($serverRequest);
         } finally {
-            // Somewhere an ob_start() is called in frontend that is not cleaned. Work around that for now.
-            ob_end_clean();
+            // TYPO3 v14 opens an implicit output buffer in Bootstrap::init(), TYPO3 v15
+            // does not. Frontend or extension code may leave further buffers open. Drop
+            // everything opened by the sub request, but never phpunit's own buffer.
+            while (ob_get_level() > $outputBufferingLevel) {
+                ob_end_clean();
+            }
             FrameworkState::pop();
         }
         return $response;

Classes/Core/Testbase.php

@@ -754,10 +754,15 @@ public function setUpBasicTypo3Bootstrap(string $instancePath): ContainerInterfa
 
         $classLoader = require $this->getPackagesPath() . '/autoload.php';
         SystemEnvironmentBuilder::run(0, SystemEnvironmentBuilder::REQUESTTYPE_CLI, false);
+        $outputBufferingLevel = ob_get_level();
         $container = Bootstrap::init($classLoader);
         // Make sure output is not buffered, so command-line output can take place and
-        // phpunit does not whine about changed output bufferings in tests.
-        ob_end_clean();
+        // phpunit does not whine about changed output bufferings in tests. TYPO3 v14
+        // opens an implicit output buffer in Bootstrap::init(), TYPO3 v15 does not.
+        // Only drop buffers opened by the bootstrap, never phpunit's own one.
+        while (ob_get_level() > $outputBufferingLevel) {
+            ob_end_clean();
+        }
 
         $this->dumpClassLoadingInformation();