[#3297262]: Theme Switcher to ensure it loads active themes as options.

modules/form_mode_theme_switcher/tests/src/Functional/FormModeManagerThemeSwitcherTest.php

+<?php
+
+namespace Drupal\Tests\form_mode_manager_theme_switcher\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+use Drupal\Tests\form_mode_manager\Functional\FormModeManagerBase;
+
+/**
+ * Tests form_mode_manager_theme_switcher.
+ *
+ * @group form_mode_manager_theme_switcher
+ *
+ * @ingroup form_mode_manager
+ */
+class FormModeManagerThemeSwitcherTest extends FormModeManagerBase {
+
+  /**
+   * Modules to install.
+   *
+   * @var array
+   */
+  protected static $modules = [
+    // Parent modules.
+    'entity_test',
+    'field',
+    'field_ui',
+    'media',
+    'block',
+    'block_content',
+    'node',
+    'user',
+    'form_mode_manager',
+    'taxonomy',
+    // Local testing requirements.
+    'form_mode_manager_theme_switcher',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Tests Theme Switcher UI availability and theme options being present.
+   *
+   * @throws \Behat\Mink\Exception\ExpectationException
+   */
+  public function testSelectSpecificTheme() {
+    $this->drupalLogin($this->rootUser);
+
+    // Theme switcher UI must be available.
+    $this->drupalGet('admin/config/content/form_mode_manager/theme-switcher');
+    $this->assertSession()->statusCodeEquals(200);
+
+    // The theme Stark must have been included in the page as an option the
+    // created form modes on node and user.
+    $node_mode_select = $this->assertSession()->selectExists(
+      "{$this->nodeFormMode->id()}_theme_type"
+    );
+    $this->assertSession()->selectExists(
+      "{$this->nodeFormMode->id()}_theme_form_mode"
+    );
+    $this->assertSession()->optionExists(
+      "{$this->nodeFormMode->id()}_theme_form_mode",
+      $this->defaultTheme
+    );
+
+    $this->assertSession()->selectExists(
+      "{$this->userFormMode->id()}_theme_type"
+    );
+    $this->assertSession()->selectExists(
+      "{$this->userFormMode->id()}_theme_form_mode"
+    );
+    $this->assertSession()->optionExists(
+      "{$this->userFormMode->id()}_theme_form_mode",
+      $this->defaultTheme
+    );
+  }
+}