CSS代码:

body {
position: relative;
}
.indicator {
position: absolute;
top: 0; right: 0; left: 0; bottom: 0;
background: linear-gradient(to right top, teal 50%, transparent 50%) no-repeat;
background-size: 100% calc(100% - 100vh);
z-index: 1;
pointer-events: none;
mix-blend-mode: darken;
}
.indicator::after {
content: '';
position: fixed;
top: 5px; bottom: 0; right: 0; left: 0;
background: #fff;
z-index: 1;
}

HTML代码:

<!-- 在<body>标签内插入指示器元素 -->
<div class="indicator"></div>

原理

使用mix-blend-mode:darken改进实现