anti-keygrabber.user.js for Mac/Emacs user

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;
  }

  var modifier = isMac ? e.metaKey : e.ctrlKey;
  // abort if the proper command/control modifier isn't pressed
  if (!modifier) {
    return;
  }

  if (e.ctrlKey) {
    switch (e.keyCode) {
      case 87: // W - close window
      case 84: // T - open tab
      case 74: // J - Japanese input switch
      case 65: // A
      case 66: // B
      case 69: // E
      case 70: // F
      case 72: // H
      case 78: // N
      case 80: // P
        e.stopImmediatePropagation();
        return;
    }
  }

  if (!isMac && e.metaKey) {
    switch (e.keyCode) {
      case 87:  // W - close window
      case 84:  // T - open tab
      case 74:  // J - Japanese input switch
      case 188: // , (comma) - open settings [mac]
      case 82:  // R - reload tab
        e.stopImmediatePropagation();
        return;
    }
  }
}, true);
})();