core/modules/block/block.module | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index f9dd787..545f504 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -7,6 +7,7 @@
 
 use Drupal\block\BlockInterface;
 use Drupal\Component\Utility\Html;
+use Drupal\Core\Cache\Cache;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Url;
 use Drupal\language\ConfigurableLanguageInterface;
@@ -306,3 +307,12 @@ function block_configurable_language_delete(ConfigurableLanguageInterface $langu
     }
   }
 }
+/**
+ * Implements hook_modules_installed().
+ */
+function block_modules_installed() {
+  // Because the Block module upon installation unconditionally overrides all
+  // HTML output by selecting a different page display variant, we must
+  // invalidate all cached HTML output.
+  Cache::invalidateTags(['rendered']);
+}