GreaseMonkey script to prevent websites from messing with my keyboard shortcuts

README.md

To install in Vivaldi, open the Extensions page, enable developer mode, and then drag & drop the file into the page.

forbid_keys.user.js

// ==UserScript==
// @name         forbid rebinding my keys
// @version      0.1
// @description  Get off my lawn!
// @author       Roland Kuhn
// @match        http://*/*
// @match        https://*/*
// ==/UserScript==

(function() {
  'use strict';

  const SHIFT = 1
  const CTRL = 2
  const ALT = 4
  const META = 8

  function prevent(ev) {
    const mod =
          (ev.shiftKey ? SHIFT : 0) |
          (ev.ctrlKey ? CTRL : 0) |
          (ev.altKey ? ALT : 0) |
          (ev.metaKey ? META : 0)
    if (ev.code == 'KeyM' && mod == (SHIFT | META)) { return true }
    if ((ev.code == 'ArrowLeft' || ev.code == 'ArrowRight') && mod == (ALT | META)) { return true }
    if (ev.code == 'KeyF' && mod == (SHIFT | META)) { return true }
    if (ev.code == 'KeyR' && mod == META) { return true }
    if (ev.code == 'KeyW' && mod == META) { return true }
    if (ev.code >= 'Digit0' && ev.code <= 'Digit9' && mod == META) { return true }
    if (ev.code == 'KeyJ' && mod == (ALT | META)) { return true }
    if (ev.code == 'KeyI' && mod == (ALT | META)) { return true }
    if ((ev.code == 'BracketLeft' || ev.code == 'BracketRight') && mod == META) { return true }
    return false
  }
  window.addEventListener('keydown', (ev) => {
    if (prevent(ev)) {
      ev.stopImmediatePropagation()
    }
  }, {capture: true})
})();