Issue #3068078 by aspilicious: Block attributes and contextual links are removed

src/View/BlockViewBuilder.php

@@ -126,6 +126,10 @@ class BlockViewBuilder {
         $block_plugin->setTitle($title);
       }
 
+      // Place the content returned by the block plugin into a 'content' child
+      // element, as a way to allow the plugin to have complete control of its
+      // properties and rendering (for instance, its own #theme) without
+      // conflicting with the properties used above.
       $build['content'] = $block_plugin->build();
 
       if ($block_plugin instanceof TitleBlockPluginInterface) {
@@ -143,6 +147,16 @@ class BlockViewBuilder {
           '#base_plugin_id' => $block_plugin->getBaseId(),
           '#derivative_plugin_id' => $block_plugin->getDerivativeId(),
         ];
+        // Semantically, the content returned by the plugin is the block, and in
+        // particular, #attributes and #contextual_links is information about
+        // the *entire* block. Therefore, we must move these properties into the
+        // top-level element.
+        foreach (['#attributes', '#contextual_links'] as $property) {
+          if (isset($build['content'][$property])) {
+            $build[$property] = $build['content'][$property];
+            unset($build['content'][$property]);
+          }
+        }
       }
     }
 

tests/src/Kernel/BlockViewBuilderTest.php

@@ -47,7 +47,9 @@ final class BlockViewBuilderTest extends KernelTestBase {
       ],
       '#theme' => 'block',
       '#id' => 'twig_tweak_test_foo',
-      '#attributes' => [],
+      '#attributes' => [
+        'id' => 'foo',
+      ],
       '#contextual_links' => [],
       '#configuration' => [
         'id' => 'twig_tweak_test_foo',
@@ -100,6 +102,11 @@ final class BlockViewBuilderTest extends KernelTestBase {
     $expected_build = [
       'content' => [
         '#markup' => 'Foo',
+        // Since the block is built without wrapper #attributes must remain in
+        // 'content' element.
+        '#attributes' => [
+          'id' => 'foo',
+        ],
         '#cache' => [
           'contexts' => ['url'],
           'tags' => ['tag_from_build'],

tests/twig_tweak_test/src/Plugin/Block/FooBlock.php

@@ -41,6 +41,9 @@ final class FooBlock extends BlockBase {
   public function build(): array {
     return [
       '#markup' => $this->getConfiguration()['content'],
+      '#attributes' => [
+        'id' => 'foo',
+      ],
       '#cache' => [
         'contexts' => ['url'],
         'tags' => ['tag_from_' . __FUNCTION__],