A userscript to prevent websites from hijacking browser keyboard shortcuts

preventKeyboardShortcutHijacking-user.js

// ==UserScript==
// @name        Prevent keyboard shortcut hijacking
// @description Prevent websites to hijack keyboard shortcuts, for example <Ctrl+F> on Discourse
// @namespace   Violentmonkey Scripts
// @match       *://*/*
// @grant       none
// @version     1.0
// @author      notDavid
// ==/UserScript==

(window.opera ? document.body : document).addEventListener('keydown', function(e) {
  //alert(e.keyCode ); //uncomment to find more keyCodes

  var OS="Unknown";
  if (navigator.appVersion.toUpperCase().indexOf("MAC")>=0) OS="macOS";
  //if (navigator.appVersion.toUpperCase().indexOf("LINUX")>=0) OS="Linux";
  //if (navigator.appVersion.toUpperCase().indexOf("X11")>=0) OS="UNIX";
  //if (navigator.appVersion.toUpperCase().indexOf("WIN")>=0) OS="Windows";
  //console.log(OS);

  // macOS uses the Command key, identified as metaKey
  // Windows and Linux use the Control key, identified as ctrlKey
  //var modifier = (OS == "macOS") ? e.metaKey : e.ctrlKey;
  var modifier_cmd = e.metaKey;
  var modifier_ctrl = e.ctrlKey;

  if (e.keyCode === 116) {
      // F5 should never be captured
      e.stopImmediatePropagation();
      return;
  }

  // abort if modifier isn't pressed
  if (modifier_cmd || modifier_ctrl) {

    if (modifier_ctrl) {
      switch (e.keyCode) {
        case 70: // f - find
          e.stopImmediatePropagation();
          return;
      }
    }

    if ((OS == "macOS") && (modifier_cmd)) {
      switch (e.keyCode) {
        case 188: // , - settings
        case 82:  // r - reload
        case 70:  // f - find (Discourse)
          e.stopImmediatePropagation();
          return;
      }
    }
  }

}, true);