         let event = new CapturedMouseEvent("", init);
          assert_equals(event.surfaceX, init.surfaceX,
                        `surfaceX with eventInitDict=${JSON.stringify(init)}`);
          assert_equals(event.surfaceY, init.surfaceY,
                        `surfaceY with eventInitDict=${JSON.stringify(init)}`);
     });
  }, "Valid surfaceX/surfaceY options are used as initial values");

  test(() => {
      let event = new CapturedMouseEvent("");
      assert_equals(event.surfaceX, -1,
        