GreaseMonkey: Prevent Web Applications From Grabbing Certain HotKeys

anti-keygrabber.user.js

// ==UserScript==
// @name anti key-grabber
// @description Prevent web apps from capturing and muting vital keyboard shortcuts
// @grant none
// @version 1.1
// ==/UserScript==
(function(){
var isMac = unsafeWindow.navigator.oscpu.toLowerCase().contains("mac os x");
unsafeWindow.document.addEventListener('keydown', function(e) {
  if (e.keyCode === 116) {
    // F5 should never be captured
    e.stopImmediatePropagation();
    return;
  }
  
  // Mac uses the Command key, identified as metaKey
  // Windows and Linux use the Control key, identified as ctrlKey
  var modifier = isMac ? e.metaKey : e.ctrlKey;
  // abort if the proper command/control modifier isn't pressed
  if (!modifier) {
    return;
  }
  
  switch (e.keyCode) {
    case 87: // W - close window
    case 84: // T - open tab
    case 76: // L - focus awesome bar
    case 74: // J - open downloads panel
      e.stopImmediatePropagation();
      return;
  }
  
  // s'more mac love
  if (!isMac) {
      return;
  }
  
  switch (e.keyCode) {
    case 188: // , (comma) - open settings [mac]
    case 82: // R - reload tab
      e.stopImmediatePropagation();
      return;
  }
}, true);
})();

Hmm.... This not appear to be working in 60.0.2.

I'm experiencing the same issue right here on github where I want to disable github's built in behavior of ctrl-b inserting bold formatting, which prevents me from simply moving point like I'm used to with my emacs keybindings.

For those like me who want to use this with TamperMonkey and the above solution doesn't work, I found a sledgehammer method: Just disable addEventListener('keydown', ...) for certain sites (inspired by this genious answer on SO):

// ==UserScript==
// @name anti keydown-event
// @description Prevent registering of keydown events
// @grant none
// @version 1.0
// @match  https://www.example.com/bitbucket/*
// @run-at document-start
// ==/UserScript==
(function(){
    'use strict';
    const orig = EventTarget.prototype.addEventListener;
    EventTarget.prototype.addEventListener = function(...args) {
        if (args[0] == 'keydown') {
            return;
        }
        return orig.apply(this, args);
    };
})();