Provide consistent order by adding secondary id sort

src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php

@@ -1064,6 +1064,14 @@ protected function prepare_items_query( $prepared_args = array(), $request = nul
 					}
 				}
 
+				// If ID is not set, try stabilizing the order. A client can see unexpected ordering when
+				// items are technically equal to one another (say all pages have a menu_order of 0), but would like to
+				// see a stable sort. This can especially help for pagination cases.
+				// See https://core.trac.wordpress.org/ticket/46294 and https://core.trac.wordpress.org/ticket/44349
+				if ( ! in_array( 'ID', $new_arg, true ) ) {
+					$new_arg[] = 'ID';
+				}
+
 				// WP_Query expects a space separated list.
 				$query_args['orderby'] = join( ' ', $new_arg );
 			} else {
@@ -1082,7 +1090,17 @@ protected function prepare_items_query( $prepared_args = array(), $request = nul
 
 				// WP_Query expects an array with orderby (database table column) keys and order ("asc", "desc") values.
 				$query_args['orderby'] = $new_arg;
+
+				// If ID is not set, try stabilizing the order. A client can see unexpected ordering when
+				// items are technically equal to one another (say all pages have a menu_order of 0), but would like to
+				// see a stable sort. This can especially help for pagination cases.
+				// See https://core.trac.wordpress.org/ticket/46294 and https://core.trac.wordpress.org/ticket/44349
+				if ( ! isset( $new_arg['ID'] ) ) {
+					$new_arg['ID'] = 'desc';
+				}
 			}
+
+			error_log( print_r( $new_arg, true ) );
 		}
 
 		return $query_args;

tests/phpunit/tests/rest-api/rest-pages-controller.php

@@ -419,12 +419,12 @@ public function test_get_items_multiple_orderby_and_order_query() {
 		$request->set_param(
 			'orderby',
 			array(
-				'id' => 'desc',
+				'id' => 'asc',
 			)
 		); //eg WP_Query uses ID internally
 		$response = rest_get_server()->dispatch( $request );
 		$data     = $response->get_data();
-		$this->assertEquals( array( $id5, $id4, $id3, $id1, $id2 ), wp_list_pluck( $data, 'id' ) );
+		$this->assertEquals( array( $id2, $id1, $id3, $id4, $id5 ), wp_list_pluck( $data, 'id' ) );
 
 		// Invalid 'orderby' should error when in array.
 		$request = new WP_REST_Request( 'GET', '/wp/v2/pages' );