ios，固定定位 会在弹出层上边

/deep/.u-mask{  
    z-index: 1073 !important;  
}  
/deep/.u-drawer-content{  
    position: fixed !important;  
    z-index: 1999;  
}

固定按钮：

.buttonBox{  
    width: 100%;  
    height: 140rpx;  
    position: fixed;  
    bottom: -6rpx;  
    left: 0;  
    background: #FFFFFF;  
    padding-top: 20rpx;  
    z-index: 9;  
    .btn{  
        width: 492rpx;  
        height: 92rpx;  
        text-align: center;  
        margin: 0 auto;  
        background: #1F77FF;  
        box-shadow: 0px 1px 12rpx #1F77FF;  
        border-radius: 8rpx;  
    }  
}