wx.createSelectorQuery返回的top值有误，不稳定,是根据什么判断的？

this.createSelectorQuery().select("#footer").boundingClientRect((res) => {

          if (!res) return;

          let top = res.top;

          let height = windowHeight - res.height;

          console.log(res)

          console.log(top)

          console.log(height)

          if (top < height) {

            console.log('进来了')

            this.setData({

              fix: true

            })

          }

        }).exec();