背景attachment和伪元素造成视觉落差效果

.bgdiv{
background-image: url(../img/nnh_n_0.jpg) ;
background-size: 100% auto;
width: 100%;
height: 768px;
position: relative;
background-color: #000;
background-attachment: fixed;
}
.bgdiv::before{
position: absolute;
content: '';
width: 100%;
height: 100%;
top: 0;
left: 0;
background: -webkit-linear-gradient(top,rgba(0,0,0,0),#000);
background: -moz-linear-gradient(top,rgba(0,0,0,0),#000);
background: linear-gradient(to bottom,rgba(0,0,0,0),#000);
}