A Greasemonkey compatible user script for indicating whether a reply in a hacker news thread is from somehow new, or someone who appeared earlier in the thread.

hn-comment-ancestory.js

// ==UserScript==
// @name     HN comment ancestory
// @version  1
// @include  https://news.ycombinator.com/item?*
// @grant    none
// ==/UserScript==

const $commentTree = document.getElementsByClassName('comment-tree')[0];
const $commentRows = $commentTree.children[0].children

const usernameStack = [];
let prevIndent = 0;

for (let i = 0, len = $commentRows.length; i < len; i++) {
  const $row = $commentRows[i];
  const $rowTable = $row.children[0].children[0];
  const indent = $rowTable.getElementsByClassName('ind')[0].children[0].width / 40;
  const $username = $rowTable.getElementsByClassName('hnuser')[0];
  const username = $username.innerText;

  let indicatorText;
  if (indent <= prevIndent) {
    usernameStack.splice(indent);
  }

  const index = usernameStack.indexOf(username);
  if (index === -1) {
    indicatorText = '*';
  } else {
    indicatorText = '↑'.repeat(indent - index);
  }

  usernameStack.push(username);
  prevIndent = indent;

  $username.innerText = indicatorText + " " + username;
}