Add preview theme suggestions.

tests/modules/ui_patterns_render_test/templates/foo.ui_patterns.yml

 foo:
   label: Foo
+  variants:
+    default:
+      label: Default

tests/src/FunctionalJavascript/UiPatternsPreviewRenderTest.php

+<?php
+
+namespace Drupal\Tests\ui_patterns\FunctionalJavascript;
+
+use Drupal\FunctionalJavascriptTests\WebDriverTestBase;
+use Drupal\Tests\ui_patterns\Traits\TwigDebugTrait;
+
+/**
+ * Test pattern preview rendering.
+ *
+ * @group ui_patterns
+ */
+class UiPatternsPreviewRenderTest extends WebDriverTestBase {
+
+  /**
+   * Disable schema validation when running tests.
+   *
+   * @var bool
+   *
+   * @todo: Fix this by providing actual schema validation.
+   */
+  protected $strictConfigSchema = FALSE;
+
+  use TwigDebugTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'ui_patterns',
+    'ui_patterns_library',
+    'ui_patterns_render_test',
+  ];
+
+  /**
+   * Tests pattern preview suggestions.
+   */
+  public function testPatternPreviewSuggestions() {
+    $assert_session = $this->assertSession();
+
+    $this->enableTwigDebugMode();
+
+    $user = $this->drupalCreateUser([], NULL, TRUE);
+    $this->drupalLogin($user);
+
+    $this->drupalGet('/patterns');
+
+    // Assert correct variant suggestions.
+    $suggestions = [
+      'pattern-foo--variant-default--preview.html.twig',
+      'pattern-foo--variant-default.html.twig',
+      'pattern-foo--preview.html.twig',
+      'pattern-foo.html.twig',
+    ];
+    foreach ($suggestions as $suggestion) {
+      $assert_session->responseContains($suggestion);
+    }
+  }
+
+}

ui_patterns.module

@@ -60,7 +60,17 @@ function ui_patterns_theme_suggestions_alter(array &$suggestions, array $variabl
  * Implements hook_ui_patterns_suggestions_alter().
  */
 function ui_patterns_ui_patterns_suggestions_alter(array &$suggestions, array $variables, PatternContext $context) {
+  // Add preview theme suggestion.
+  if ($context->isOfType('preview')) {
+    $suggestions[] = $variables['theme_hook_original'] . '__preview';
+  }
+
   if (!empty($variables['variant'])) {
     $suggestions[] = $variables['theme_hook_original'] . '__variant_' . $variables['variant'];
+
+    // Add variant preview theme suggestion.
+    if ($context->isOfType('preview')) {
+      $suggestions[] = $variables['theme_hook_original'] . '__variant_' . $variables['variant'] . '__preview';
+    }
   }
 }