            `surfaceX with implicit eventInitDict={}`);
      assert_equals(event.surfaceY, -1,
                    `surfaceY with implicit eventInitDict={}`);

      [
          {},
          {surfaceX: -1},
          {surfaceY: -1},
      ].forEach(init => {
          let event = new CapturedMouseEvent("", init);
          assert_equals(event.surfaceX, -1,
                        `surfaceX with eventInitDict=${JSON.stringify(init)}`);
          assert_equals(event.surfaceY, -1,
                        `su