Test newly created variations are returned by the endpoint

tests/phpunit/tests/rest-api/rest-global-styles-controller.php

@@ -602,6 +602,55 @@ public function test_update_item_invalid_styles_css() {
 		$this->assertErrorResponse( 'rest_custom_css_illegal_markup', $response, 400 );
 	}
 
+	/**
+	 * Tests the submission of a custom block style variation that was defined
+	 * within a theme style variation and wouldn't be registered at the time
+	 * of saving via the API.
+	 *
+	 * @covers WP_REST_Global_Styles_Controller_Gutenberg::update_item
+	 * @ticket 61312
+	 */
+	public function test_update_item_with_custom_block_style_variations() {
+		wp_set_current_user( self::$admin_id );
+		if ( is_multisite() ) {
+			grant_super_admin( self::$admin_id );
+		}
+
+		$group_variations = array(
+			'fromThemeStyleVariation' => array(
+				'color' => array(
+					'background' => '#ffffff',
+					'text'       => '#000000',
+				),
+			),
+		);
+
+		$request = new WP_REST_Request( 'PUT', '/wp/v2/global-styles/' . self::$global_styles_id );
+		$request->set_body_params(
+			array(
+				'styles' => array(
+					'blocks' => array(
+						'variations' => array(
+							'fromThemeStyleVariation' => array(
+								'blockTypes' => array( 'core/group', 'core/columns' ),
+								'color'      => array(
+									'background' => '#000000',
+									'text'       => '#ffffff',
+								),
+							),
+						),
+						'core/group' => array(
+							'variations' => $group_variations,
+						),
+					),
+				),
+			)
+		);
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+		$this->assertSame( $group_variations, $data['styles']['blocks']['core/group']['variations'] );
+	}
+
 	/**
 	 * @doesNotPerformAssertions
 	 */