分析需求

如何得知用户当前滚动页面的距离并且通知顶部进度条？

如此思考则陷入了传统思维，进度条就是进度条，接收页面滚动距离，改变宽度。如果页面滚动和进度条是一个整体呢。

实现需求

线性渐变
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.scroll{
    margin: 1rem 0;
    height: 20rem;
    border: 1px dashed #cdcdcd;
    overflow: auto;
}
.wrap{
    margin-top: 8px;
    padding: 1rem;
    position: relative;
}
.wrap blockquote,.wrap figure.highlight{
    margin: 0!important;
    padding: 0;
}
.wrap blockquote p {
    padding-left: 1rem;
}
.wrap figure.highlight,.wrap pre{
    background: transparent!important
}
.wrap::after {
    content: "";
    position: fixed;
    top: 3.15rem;
    left: 0;
    width: 100%;
    height: calc(20rem - .5rem);
    background: #fff;
    z-index: -1;
}
.wrap::before {
    content: "";
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-image: linear-gradient(to right top, #ffcc00 50%, #eee 50%);
    background-size: calc(100% + 5px) calc(100% - 20.2rem + 8px);
    background-repeat: no-repeat;
    z-index: -2;
}
2020-07-31CSS