Grant webapps full keyboard access to enable use of additional shortcuts; useful with Apache Guacamole remote sessions.

FullScreenGrabKeyboard.user.js

// ==UserScript==
// @name         Full-screen Grab Keyboard
// @version      0.1
// @description  Lock the keyboard in full-screen mode, granting webapps access to additional key shortcuts
// @match        https://SITE_URL_HERE/*
// @grant        none
// @noframes
// @author       xenago
// ==/UserScript==

// Notes:
// - Only compatible with Chromium-based browsers
// - Full-screen is required for this to work, it does not function outside that mode

// References:
// Original author toff's version: https://gist.github.com/toff/ab1ba901dc663297af1fb4b8e630d899
// https://developer.mozilla.org/en-US/docs/Web/API/Keyboard/lock#browser_compatibility
// https://chromestatus.com/feature/5642959835889664
// https://wicg.github.io/keyboard-lock/
// https://github.com/WICG/keyboard-lock

(function() {
    'use strict';
    // Create a button to go full-screen.
    let fsButtonElem = document.createElement("input");
    fsButtonElem.type = "button";
    fsButtonElem.value = "🖵";
    fsButtonElem.style.position = "absolute";
    fsButtonElem.style.top = "8px";
    fsButtonElem.style.left = "24px";
    fsButtonElem.style.opacity = "40%";
    fsButtonElem.style.padding = "3px 4px 0px 4px";
    fsButtonElem.style.zIndex = 10000;
    fsButtonElem.style.fontSize = "11px";
    fsButtonElem.isMoving = false;
    fsButtonElem.addEventListener("click", function() {
        if (!fsButtonElem.isMoving) {
            if (!document.fullscreenElement) {
                document.body.requestFullscreen();
            } else {
                document.exitFullscreen();
            }
        }
        fsButtonElem.isMoving = false;
    });
    document.body.appendChild(fsButtonElem);
    
    // Lock the keyboard once in full-screen mode
    document.addEventListener("fullscreenchange", event => {
        if (document.fullscreenElement) {
            if (navigator.keyboard) {
                console.log("Locking keyboard.");
                navigator.keyboard.lock();
            } else {
                console.error("Locking keyboard is not supported.");
            }
        }
    });

})();

Note: this is extremely useful with Apache Guacamole, to ensure actions like Alt+Tab get passed through to a remote session.