Decode ROT13 messages in Twitch chat via CTRL + double-click

deRot13.user.js

// ==UserScript==
// @name         Twitch Chat ROT13 Messages
// @namespace    https://gist.github.com/awsr
// @version      2.0.0
// @description  Decode ROT13 messages in Twitch chat via CTRL + double-click
// @author       Tenosis
// @downloadURL  https://gist.github.com/awsr/ad422b3212a08813977c61e7d61ae94f/raw/deRot13.user.js
// @match        https://www.twitch.tv/*
// @grant        none
// ==/UserScript==

(function () {
  'use strict';

  // INFO:
  // Use CTRL + double-click to toggle ROT13 state.

  // CSS Styles ====
  const scriptCSS = document.createElement("style");
  scriptCSS.innerText = ".userscript-rot13>div:first-child::before{display:block;width:4px;height:100%;background-color:#debb1f;content:'';position:absolute;left:-8px;border-radius:3px 0 0 3px;}";
  document.head.append(scriptCSS);

  // Functions =====
  function rot13(text) {
    return text.replace(/[a-z]/gi, letter => String.fromCharCode(letter.charCodeAt(0) + (letter.toLowerCase() <= 'm' ? 13 : -13)));
  }

  function decoder(event) {
    // Only check if control key is pressed
    if (event.ctrlKey) {
      const chatLine = event.target.closest(".chat-line__message");
      if (chatLine) {
        const textTarget = chatLine.querySelectorAll(".text-fragment");
        if (textTarget.length) {
          // Process multiple segments in case of mid-sentence special formatting
          textTarget.forEach((segment) => { segment.textContent = rot13(segment.textContent) });
          // Mark line to indicate it has been changed
          chatLine.classList.toggle("userscript-rot13");
          // Remove any text selection from double-clicking
          getSelection().removeAllRanges();
          // De-focus chat line
          chatLine.blur();
        }
      }
    }
  }

  // Start =========
  document.documentElement.addEventListener("dblclick", decoder);
})();