Issue #1895984 by sdboyer: Improve new-style controller selection by relying on ContentNegotiation.

core/lib/Drupal/Core/CoreBundle.php

@@ -199,6 +199,7 @@ class CoreBundle extends Bundle {
       ->addTag('route_filter');
 
     $container->register('router_processor_subscriber', 'Drupal\Core\EventSubscriber\RouteProcessorSubscriber')
+      ->addArgument(new Reference('content_negotiation'))
       ->addTag('event_subscriber');
     $container->register('router_listener', 'Symfony\Component\HttpKernel\EventListener\RouterListener')
       ->addArgument(new Reference('router'))

core/lib/Drupal/Core/EventSubscriber/RouteProcessorSubscriber.php

@@ -16,12 +16,19 @@ use Symfony\Component\HttpKernel\Log\LoggerInterface;
 use Symfony\Component\HttpKernel\KernelEvents;
 use Symfony\Component\Routing\Matcher\UrlMatcherInterface;
 use Symfony\Component\Routing\Exception\ResourceNotFoundException;
+use Drupal\Core\ContentNegotiation;
 
 /**
  * Listener to process request controller information.
  */
 class RouteProcessorSubscriber implements EventSubscriberInterface {
 
+  protected $negotiation;
+
+  public function __construct(ContentNegotiation $negotiation) {
+    $this->negotiation = $negotiation;
+  }
+
   /**
    * Sets a default controller for a route if one was not specified.
    *
@@ -31,7 +38,7 @@ class RouteProcessorSubscriber implements EventSubscriberInterface {
   public function onRequestSetController(GetResponseEvent $event) {
     $request = $event->getRequest();
 
-    if (!$request->attributes->has('_controller') && $request->attributes->has('_content')) {
+    if (!$request->attributes->has('_controller') && $this->negotiation->getContentType($request) === 'html') {
       $request->attributes->set('_controller', '\Drupal\Core\HtmlPageController::content');
     }
   }