Issue #3482208 by pdureau: Fix attributes merging

src/Element/ComponentElementAlter.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace Drupal\ui_patterns\Element;
 
+use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Plugin\Component;
 use Drupal\Core\Security\TrustedCallbackInterface;
 use Drupal\Core\Theme\ComponentPluginManager;
@@ -120,7 +121,13 @@ class ComponentElementAlter implements TrustedCallbackInterface {
     if (is_a($element["#attributes"], '\Drupal\Core\Template\Attribute')) {
       $element["#attributes"] = $element["#attributes"]->toArray();
     }
-    $element["#props"]["attributes"] = array_merge(
+    // Like \Drupal\Core\Template\Attribute::merge(), we use
+    // NestedArray::mergeDeep().
+    // This function is similar to PHP's array_merge_recursive() function, but
+    // it handles non-array values differently. When merging values that are
+    // not both arrays, the latter value replaces the former rather than
+    // merging with it.
+    $element["#props"]["attributes"] = NestedArray::mergeDeep(
       $element["#attributes"],
       $element["#props"]["attributes"]
     );

src/Plugin/UiPatterns/Source/AttributesWidget.php

@@ -29,7 +29,7 @@ class AttributesWidget extends SourcePluginBase {
     // possible anymore because SDC will not validate it against the prop
     // type schema.
     $value = $this->getSetting('value');
-    if (!is_string($value)) {
+    if (empty($value) || !is_string($value)) {
       return [];
     }
     return $this->convertStringToAttributesMapping($value);

src/Plugin/UiPatterns/Source/ClassAttributeWidget.php

@@ -14,7 +14,7 @@ use Drupal\ui_patterns\SourcePluginPropValue;
  */
 #[Source(
   id: 'class_attribute',
-  label: new TranslatableMarkup('Class attribute'),
+  label: new TranslatableMarkup('HTML classes'),
   description: new TranslatableMarkup('A space-separated list of HTML classes.'),
   prop_types: ['attributes']
 )]
@@ -28,6 +28,9 @@ class ClassAttributeWidget extends SourcePluginPropValue {
     // possible anymore because SDC will not validate it against the prop
     // type schema.
     $value = parent::getPropValue();
+    if (empty($value)) {
+      return [];
+    }
     return is_string($value) ? $this->convertStringToAttributesMapping($value) : [];
   }