autoscroll.ts

import { beforeUpdate, afterUpdate } from 'svelte';

let autoscroll = true;
let prevHeight = 0;
let footer = document.querySelector('footer');

beforeUpdate(() => {
  prevHeight = document.body.scrollHeight;
  // autoscroll if the footer is visible
  let footerTop = footer?.offsetTop ?? document.body.scrollHeight;
  autoscroll = window.scrollY + window.innerHeight >= footerTop;
  console.log(footerTop, window.scrollY, window.innerHeight);
});

afterUpdate(() => {
  if (autoscroll) {
    let scrollDiff = document.body.scrollHeight - prevHeight;
    window.scrollBy({behavior:"smooth", top: scrollDiff});
  }
});