body {
    background: linear-gradient(to right top, red 50%, #fff 50%) no-repeat;
    background-size: 100% calc(100% - 100vh + 5px);
}

body::after {
    content: '';
    position: fixed;
    top: 5px;
    right: 0;
    bottom: 0;
    left: 0;
    background-color: #fff;
    z-index: -1;
}