doc/themes/learn4/app/utils/progress.js

const h = document.documentElement;
const b = document.body;
const st = 'scrollTop';
const sh = 'scrollHeight';
const progress = document.querySelector('.progress');
let scroll;

document.addEventListener('scroll', function() {
    scroll = (h[st] || b[st]) / ((h[sh] || b[sh]) - h.clientHeight) * 100;
    progress.style.setProperty('--scroll', scroll + '%');
});