fix(login): allow Cornerstone previews

inc/checkout/class-checkout-pages.php

@@ -705,6 +705,7 @@ public function maybe_redirect_to_admin_panel(): void {
 				'elementor-preview', // Elementor
 				'brizy-edit',        // Brizy
 				'brizy-edit-iframe', // Brizy
+				'cs_preview_state',  // Cornerstone
 			],
 			$custom_login_page,
 			$post,

tests/WP_Ultimo/Checkout/Checkout_Pages_Test.php

@@ -1179,7 +1179,7 @@
 
 		$called = false;
 
 		$callback = function ($url, $key, $user_login, $user_data) use (&$called) {
 			$called = true;
 			// Integration overrides the destination but keeps host.
 			return add_query_arg('via', 'integration', $url);
@@ -1995,6 +1995,30 @@
 		$this->assertTrue(true);
 	}
 
+	/**
+	 * Test maybe_redirect_to_admin_panel allows Cornerstone preview requests.
+	 */
+	public function test_maybe_redirect_to_admin_panel_allows_cornerstone_preview(): void {
+
+		$user_id = self::factory()->user->create(['role' => 'subscriber']);
+		wp_set_current_user($user_id);
+
+		$login_page_id = self::factory()->post->create(['post_type' => 'page']);
+		wu_save_setting('default_login_page', $login_page_id);
+
+		global $post;
+		$post = get_post($login_page_id);
+
+		$_REQUEST['cs_preview_state'] = 'preview-state';
+
+		$this->pages->maybe_redirect_to_admin_panel();
+
+		unset($_REQUEST['cs_preview_state']);
+		wp_set_current_user(0);
+
+		$this->assertTrue(true);
+	}
+
 	/**
 	 * Test maybe_redirect_to_admin_panel exclusion list filter is applied.
 	 */
@@ -2010,7 +2034,7 @@
 		$post = get_post($login_page_id);
 
 		// Add a custom exclusion param via filter.
 		add_filter('wu_maybe_redirect_to_admin_panel_exclusion_list', function ($list) {
 			$list[] = 'my_custom_builder';
 			return $list;
 		});