code: function getPermissionY(dragTd, evt) {
          try {
              var top = domUtils.getXY(dragTd).y,
                  mousePosY = mouseCoords(evt).y;
              return mousePosY < top ? top : mousePosY;
          } catch (e) {
              showError(e);
          }
      }
docs: 获取拖动时允许的Y轴坐标