size event at this moment.
      iframe.contentDocument.body.clientWidth;

      requestAnimationFrame(t.step_func_done(() => {
        // it's uncontrollable whether the test will run
        // with classic or overlay scrollbars in the case of Mac,
        // so the check has to be conditional.
        const width_changed = iframe.contentWindow.visualViewport.width !== originalVisualViewportWidth;
        assert_equals(resize_event_count, width_changed ? 1 : 0);
      }));
    }
  }, "the appearance of cl