.../render_placeholder_message_test.info.yml       |   6 ++
 .../render_placeholder_message_test.routing.yml    |  24 +++++
 .../src/RenderPlaceholderMessageTestController.php | 100 +++++++++++++++++++++
 .../Functional/Render/PlaceholderMessageTest.php   |  58 ++++++++++++
 4 files changed, 188 insertions(+)

diff --git a/core/modules/system/tests/modules/render_placeholder_message_test/render_placeholder_message_test.info.yml b/core/modules/system/tests/modules/render_placeholder_message_test/render_placeholder_message_test.info.yml
new file mode 100644
index 0000000..532695b
--- /dev/null
+++ b/core/modules/system/tests/modules/render_placeholder_message_test/render_placeholder_message_test.info.yml
@@ -0,0 +1,6 @@
+name: 'Placeholder setting a message test'
+type: module
+description: 'Support module for PlaceholderMessageTest.'
+package: Testing
+version: VERSION
+core: 8.x
diff --git a/core/modules/system/tests/modules/render_placeholder_message_test/render_placeholder_message_test.routing.yml b/core/modules/system/tests/modules/render_placeholder_message_test/render_placeholder_message_test.routing.yml
new file mode 100644
index 0000000..6734cf3
--- /dev/null
+++ b/core/modules/system/tests/modules/render_placeholder_message_test/render_placeholder_message_test.routing.yml
@@ -0,0 +1,24 @@
+render_placeholder_message_test.first:
+  path: '/render_placeholder_message_test/first'
+  defaults:
+    _controller: '\Drupal\render_placeholder_message_test\RenderPlaceholderMessageTestController::messagesPlaceholderFirst'
+  requirements:
+    _access: 'TRUE'
+render_placeholder_message_test.middle:
+  path: '/render_placeholder_message_test/middle'
+  defaults:
+    _controller: '\Drupal\render_placeholder_message_test\RenderPlaceholderMessageTestController::messagesPlaceholderMiddle'
+  requirements:
+    _access: 'TRUE'
+render_placeholder_message_test.last:
+  path: '/render_placeholder_message_test/last'
+  defaults:
+    _controller: '\Drupal\render_placeholder_message_test\RenderPlaceholderMessageTestController::messagesPlaceholderLast'
+  requirements:
+    _access: 'TRUE'
+render_placeholder_message_test.queued:
+  path: '/render_placeholder_message_test/queued'
+  defaults:
+    _controller: '\Drupal\render_placeholder_message_test\RenderPlaceholderMessageTestController::queuedMessages'
+  requirements:
+    _access: 'TRUE'
diff --git a/core/modules/system/tests/modules/render_placeholder_message_test/src/RenderPlaceholderMessageTestController.php b/core/modules/system/tests/modules/render_placeholder_message_test/src/RenderPlaceholderMessageTestController.php
new file mode 100644
index 0000000..7aeaadb
--- /dev/null
+++ b/core/modules/system/tests/modules/render_placeholder_message_test/src/RenderPlaceholderMessageTestController.php
@@ -0,0 +1,100 @@
+<?php
+
+namespace Drupal\render_placeholder_message_test;
+use Drupal\Core\Render\RenderContext;
+use Symfony\Component\DependencyInjection\ContainerAwareInterface;
+use Symfony\Component\DependencyInjection\ContainerAwareTrait;
+
+class RenderPlaceholderMessageTestController implements ContainerAwareInterface {
+
+  use ContainerAwareTrait;
+
+  /**
+   * @return array
+   */
+  public function messagesPlaceholderFirst() {
+    return $this->build([
+      '<drupal-render-placeholder callback="Drupal\Core\Render\Element\StatusMessages::renderMessages" arguments="0" token="a8c34b5e"></drupal-render-placeholder>',
+      '<drupal-render-placeholder callback="\Drupal\render_placeholder_message_test\RenderPlaceholderMessageTestController::setAndLogMessage" arguments="0=P1" token="0546ada3"></drupal-render-placeholder>',
+      '<drupal-render-placeholder callback="\Drupal\render_placeholder_message_test\RenderPlaceholderMessageTestController::setAndLogMessage" arguments="0=P2" token="83d2df0d"></drupal-render-placeholder>',
+    ]);
+  }
+
+  /**
+   * @return array
+   */
+  public function messagesPlaceholderMiddle() {
+    return $this->build([
+      '<drupal-render-placeholder callback="\Drupal\render_placeholder_message_test\RenderPlaceholderMessageTestController::setAndLogMessage" arguments="0=P1" token="0546ada3"></drupal-render-placeholder>',
+      '<drupal-render-placeholder callback="Drupal\Core\Render\Element\StatusMessages::renderMessages" arguments="0" token="a8c34b5e"></drupal-render-placeholder>',
+      '<drupal-render-placeholder callback="\Drupal\render_placeholder_message_test\RenderPlaceholderMessageTestController::setAndLogMessage" arguments="0=P2" token="83d2df0d"></drupal-render-placeholder>',
+    ]);
+  }
+
+  /**
+   * @return array
+   */
+  public function messagesPlaceholderLast() {
+    return $this->build([
+      '<drupal-render-placeholder callback="\Drupal\render_placeholder_message_test\RenderPlaceholderMessageTestController::setAndLogMessage" arguments="0=P1" token="0546ada3"></drupal-render-placeholder>',
+      '<drupal-render-placeholder callback="\Drupal\render_placeholder_message_test\RenderPlaceholderMessageTestController::setAndLogMessage" arguments="0=P2" token="83d2df0d"></drupal-render-placeholder>',
+      '<drupal-render-placeholder callback="Drupal\Core\Render\Element\StatusMessages::renderMessages" arguments="0" token="a8c34b5e"></drupal-render-placeholder>',
+    ]);
+  }
+
+  /**
+   * @return array
+   */
+  public function queuedMessages() {
+    return ['#type' => 'status_messages'];
+  }
+
+  /**
+   * @return array
+   */
+  protected function build(array $placeholder_order) {
+    $build = [];
+    $build['messages'] = ['#type' => 'status_messages'];
+    $build['p1'] = [
+      '#lazy_builder' => ['\Drupal\render_placeholder_message_test\RenderPlaceholderMessageTestController::setAndLogMessage', ['P1']],
+      '#create_placeholder' => TRUE,
+    ];
+    $build['p2'] = [
+      '#lazy_builder' => ['\Drupal\render_placeholder_message_test\RenderPlaceholderMessageTestController::setAndLogMessage', ['P2']],
+      '#create_placeholder' => TRUE,
+    ];
+
+    /** @var \Drupal\Core\Render\RendererInterface $renderer */
+    $renderer = $this->container->get('renderer');
+    $renderer->executeInRenderContext(new RenderContext(), function () use (&$build, $renderer) {
+      return $renderer->render($build, FALSE);
+    });
+
+
+    $reordered = [];
+    foreach ($placeholder_order as $placeholder) {
+      $reordered[$placeholder] = $build['#attached']['placeholders'][$placeholder];
+    }
+    $build['#attached']['placeholders'] = $reordered;
+
+    return $build;
+  }
+
+  /**
+   * #lazy_builder callback; sets and prints a message.
+   *
+   * @param string $message
+   *   The message to send.
+   *
+   * @return array
+   *   A renderable array containing the message.
+   */
+  public static function setAndLogMessage($message) {
+    // Set message.
+    drupal_set_message($message);
+
+    // Print which message is expected.
+    return ['#markup' => '<p class="logged-message">Message: ' . $message . '</p>'];
+  }
+
+}
diff --git a/core/modules/system/tests/src/Functional/Render/PlaceholderMessageTest.php b/core/modules/system/tests/src/Functional/Render/PlaceholderMessageTest.php
new file mode 100644
index 0000000..8ededd1
--- /dev/null
+++ b/core/modules/system/tests/src/Functional/Render/PlaceholderMessageTest.php
@@ -0,0 +1,58 @@
+<?php
+
+namespace Drupal\Tests\system\Functional\Render;
+
+use Drupal\Core\Url;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Functional test verifying that messages set in placeholders always appear.
+ *
+ * @group Render
+ */
+class PlaceholderMessageTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = ['render_placeholder_message_test'];
+
+  /**
+   * Test rendering of message placeholder.
+   */
+  public function testMessagePlaceholder() {
+    $messages_markup = '<div role="contentinfo" aria-label="Status message" class="messages';
+
+    $test_routes = [
+      // Messages placeholder rendered first.
+      'render_placeholder_message_test.first',
+      // Messages placeholder rendered after one, before another.
+      'render_placeholder_message_test.middle',
+      // Messages placeholder rendered last.
+      'render_placeholder_message_test.last',
+    ];
+
+    $assert = $this->assertSession();
+    foreach ($test_routes as $route) {
+      // Verify that we start off with zero messages queued.
+      $this->drupalGet(Url::fromRoute('render_placeholder_message_test.queued'));
+      $assert->responseNotContains($messages_markup);
+
+      // Verify the test case at this route behaves as expected.
+      $this->drupalGet(Url::fromRoute($route));
+      $assert->elementContains('css', 'p.logged-message:nth-of-type(1)', 'Message: P1');
+      $assert->elementContains('css', 'p.logged-message:nth-of-type(2)', 'Message: P2');
+      $assert->responseContains($messages_markup);
+      $assert->elementExists('css', 'div.messages .messages__list');
+      $assert->elementContains('css', 'div.messages .messages__list .messages__item:nth-of-type(1)', 'P1');
+      $assert->elementContains('css', 'div.messages .messages__list .messages__item:nth-of-type(2)', 'P2');
+
+      // Verify that we end with all messages printed, hence again zero queued.
+      $this->drupalGet(Url::fromRoute('render_placeholder_message_test.queued'));
+      $assert->responseNotContains($messages_markup);
+    }
+  }
+
+}