Issue #3173946 by Charlie ChX Negyesi, Chi: drupal_region() overcaches empty

src/View/RegionViewBuilder.php

@@ -81,7 +81,10 @@ class RegionViewBuilder {
 
     $build = [];
 
-    $cache_metadata = new CacheableMetadata();
+    $entity_type = $this->entityTypeManager->getDefinition('block');
+    $cache_metadata = (new CacheableMetadata())
+      ->addCacheTags($entity_type->getListCacheTags())
+      ->addCacheContexts($entity_type->getListCacheContexts());
 
     /** @var \Drupal\block\BlockInterface[] $blocks */
     foreach ($blocks as $id => $block) {
@@ -102,8 +105,8 @@ class RegionViewBuilder {
     if ($build) {
       $build['#region'] = $region;
       $build['#theme_wrappers'] = ['region'];
-      $cache_metadata->applyTo($build);
     }
+    $cache_metadata->applyTo($build);
 
     return $build;
   }

tests/src/Kernel/RegionViewBuilderTest.php

@@ -63,7 +63,14 @@ final class RegionViewBuilderTest extends KernelTestBase {
 
     $build = $view_builder->build('sidebar_first');
     // The build should be empty because 'stable' is not a default theme.
-    self::assertSame([], $build);
+    $expected_build = [
+      '#cache' => [
+        'contexts' => [],
+        'tags' => ['config:block_list'],
+        'max-age' => Cache::PERMANENT,
+      ],
+    ];
+    self::assertSame($expected_build, $build);
 
     // Specify the theme name explicitly.
     $build = $view_builder->build('sidebar_first', 'stable');
@@ -103,6 +110,7 @@ final class RegionViewBuilderTest extends KernelTestBase {
         'contexts' => ['user'],
         'tags' => [
           'config:block.block.public_block',
+          'config:block_list',
           'tag_for_private_block',
           'tag_for_public_block',
         ],