Resolve #3519543 "Bug sample entity"

Closes #3519543

src/Element/ComponentForm.php

@@ -6,6 +6,7 @@ use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Plugin\Context\Context;
 use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\Plugin\Context\EntityContext;
 use Drupal\Core\Render\RenderContext;
 
 /**
@@ -161,6 +162,17 @@ class ComponentForm extends ComponentFormBase {
       $contextComponentDefinition = ContextDefinition::create('string');
       $element['#source_contexts']['component_id'] = new Context($contextComponentDefinition, $component_id);
     }
+    // Check the root entity if it is a sample entity
+    // and pass this sample entity down the render tree.
+    // Plugins can use this determine if they running in sample mode.
+    if (isset($element['#source_contexts']['entity']) &&
+      // Pass the root entity down.
+      !isset($element['#source_contexts']['sample:entity'])) {
+      $entity = $element['#source_contexts']['entity']->getContextValue();
+      if ($entity->isSampleEntity) {
+        $element['#source_contexts']['sample:entity'] = $element['#source_contexts']['entity'];
+      }
+    }
     if (empty($initial_component_id)) {
       $component_selector_form = array_merge(self::buildComponentSelectorForm(
         $wrapper_id,