slack.fullname-with-username.user.js

// ==UserScript==
// @name         Slack: Full Name (@username)
// @namespace    http://hatari.st/
// @version      0.1
// @description  nuff said
// @author       igor@hatari.st
// @match        https://*.slack.com/messages/*
// ==/UserScript==

(function() {
  var timeout;

  var observer = new MutationObserver(function(mutations) {
    mutations.forEach(function(mutation) {
      if (mutation.addedNodes.length !== 0) {
        for(var i = 0; i < mutation.addedNodes.length; i++) {
          if (mutation.addedNodes[i].className == 'day_container' || mutation.addedNodes[i].nodeName == 'TS-MESSAGE') {
            addUsernames();
            break;
          }
        }
      }
    });
  });

  function addUsernames() {
    if (!document.body.classList.contains('loading')) {
      var senders = document.querySelectorAll('.message_sender');
      for (var i = 0; i < senders.length; i++) {
        if (senders[i].textContent.indexOf(')') === -1) {
          senders[i].textContent = senders[i].textContent + " (@" + senders[i].getAttribute('href').split('/')[2] + ")";
        }
      }
    }
  }

  var target = document.querySelector('#msgs_div');
  observer.observe(target, {childList: true, subtree: true});
  window.clearInterval(timeout);
})();