.../Drupal/Core/Render/Element/StatusMessages.php  | 23 ++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/core/lib/Drupal/Core/Render/Element/StatusMessages.php b/core/lib/Drupal/Core/Render/Element/StatusMessages.php
index c2fa305..485718d 100644
--- a/core/lib/Drupal/Core/Render/Element/StatusMessages.php
+++ b/core/lib/Drupal/Core/Render/Element/StatusMessages.php
@@ -37,6 +37,15 @@ public function getInfo() {
   /**
    * #pre_render callback to generate a placeholder.
    *
+   * Ensures the same token is used for all instances, hence resulting in the
+   * same placeholder for all places rendering the status messages for this
+   * request (e.g. in multiple blocks). This ensures we can put the rendered
+   * messages in all placeholders in one go.
+   * Also ensures the same context key is used for the #post_render_cache
+   * property, this ensures that if status messages are rendered multiple times,
+   * their individual (but identical!) #post_render_cache properties are merged,
+   * ensuring the callback is only invoked once.
+   *
    * @see ::renderMessages()
    *
    * @param array $element
@@ -49,18 +58,24 @@ public static function generatePlaceholder(array $element) {
     $plugin_id = 'status_messages';
 
     $callback = get_class() . '::renderMessages';
+    try {
+      $hash_salt = Settings::getHashSalt();
+    }
+    catch (\RuntimeException $e) {
+      // Status messages are also shown during the installer, at which time no
+      // hash salt is defined yet.
+      $hash_salt = Crypt::randomBytes(8);
+    }
+    $key = $plugin_id . $element['#display'];
     $context = [
       'display' => $element['#display'],
+      'placeholder_token' => Crypt::hmacBase64($key, $hash_salt),
     ];
     $element['messages_placeholder'] = [
       '#pre_render_cache' => [
         $callback => $context,
       ],
       '#create_placeholder' => TRUE,
-      // Make it uncacheable.
-      '#cache' => [
-        'max-age' => 0,
-      ],
     ];
 
     return $element;