fixed的left:50%,漂浮

.floor-box{width: 44px; border: 1px solid #ccc; position: fixed; left: 50%; z-index: 1000}

漂浮距离，距中间50%

.floor-box{width: 44px; border: 1px solid #ccc; position: fixed; left: 50%; margin-left: 620px; bottom: 30px; background: #fff; z-index: 1000}

漂浮距离：距离中间50%，在减去620px