src/Entity/Builder/PrivateMessageThreadViewBuilder.php

@@ -112,7 +112,7 @@ class PrivateMessageThreadViewBuilder extends EntityViewBuilder {
       $build['#suffix'] = '</div></div>';
     }
     else {
-      $build['#prefix'] = '<div id="private-message-thread-' . $entity->id() . '" class="' . implode(' ', $classes) . '" data-thread-id="' . $entity->id() . '" data-last-update="' . $entity->get('updated')->value . '">';
+      $build['#prefix'] = '<div class="' . implode(' ', $classes) . '" data-thread-id="' . $entity->id() . '" data-last-update="' . $entity->get('updated')->value . '">';
       $build['#suffix'] = '</div>';
     }
 

src/Form/PrivateMessageForm.php

@@ -100,11 +100,11 @@ class PrivateMessageForm extends ContentEntityForm {
   protected $formBuilder;
 
   /**
-   * The private message configuration.
+   * The message configuration.
    *
    * @var \Drupal\Core\Config\ImmutableConfig
    */
-  private ImmutableConfig $config;
+  protected ImmutableConfig $config;
 
   /**
    * Constructs a PrivateMessageForm object.

tests/src/Functional/PrivateMessageThreadMembersTest.php

@@ -156,4 +156,18 @@ class PrivateMessageThreadMembersTest extends BrowserTestBase {
     $this->assertSession()->elementTextContains('css', '.private-message-recipients article:nth-of-type(2) .username', $this->userC->getDisplayName());
   }
 
+  /**
+   * Regression test for multiple add more clicks.
+   */
+  public function testMultipleAddMoreClicks(): void {
+    $this->drupalLogin($this->userA);
+
+    $this->drupalGet('/private-message/create');
+
+    $this->getSession()->getPage()->pressButton('edit-members-add-more');
+    $this->assertSession()->statusCodeEquals(200);
+    $this->getSession()->getPage()->pressButton('edit-members-add-more');
+    $this->assertSession()->statusCodeEquals(200);
+  }
+
 }

tests/src/FunctionalJavascript/PrivateMessageJavascriptTest.php

@@ -123,7 +123,7 @@ class PrivateMessageJavascriptTest extends WebDriverTestBase {
     // Test loading threads by clicking inbox links.
     $this->drupalGet('/private-messages');
     foreach ($threads_map as $threadId => $expectedUser) {
-      $this->click("#private-message-thread-$threadId .private-message-inbox-thread-link");
+      $this->click(".private-message-thread-inbox[data-thread-id=$threadId] .private-message-inbox-thread-link");
       $this->assertSession()->assertWaitOnAjaxRequest();
       $this->assertSession()->elementTextContains('css', '#private-message-page .private-message-recipients article:nth-of-type(1) .username', $expectedUser->getDisplayName());
     }