Issue #3473490 by fago: Fix JSON serialization of markup only slots

src/CustomElementNormalizer.php

@@ -4,7 +4,6 @@ namespace Drupal\custom_elements;
 
 use Drupal\Component\Render\MarkupInterface;
 use Drupal\Core\Render\BubbleableMetadata;
-use Drupal\Core\Template\Attribute;
 use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
 
 /**
@@ -117,16 +116,12 @@ class CustomElementNormalizer implements NormalizerInterface {
         // Handle slots set via nested custom element and markup.
         if (!empty($slot['content']) && $slot['content'] instanceof CustomElement) {
           $slot_data[$index] = $this->normalizeCustomElement($slot['content'], $cache_metadata);
+          // Remove possible doubled slot attributes.
+          unset($slot_data[$index]['slot']);
         }
         elseif ($slot['content'] instanceof MarkupInterface) {
-          $slot_data[$index]['content'] = (string) $slot['content'];
-          if (!empty($slot['attributes']) && $slot['attributes'] instanceof Attribute) {
-            $slot_data[$index] = array_merge($slot_data[$index], $slot['attributes']->toArray());
-          }
+          $slot_data[$index] = (string) $slot['content'];
         }
-
-        // Remove possible doubled slot attributes.
-        unset($slot_data[$index]['slot']);
       }
 
       if ($element->hasSlotNormalizationStyle($slot_key, CustomElement::NORMALIZE_AS_SINGLE_VALUE)) {