Tamperscript script to add a fullscreen icon to WaniKani for true fullscreen on MacOS

wanikani-fullscreen_icon.user.js

// ==UserScript==
// @name         WaniKani Fullscreen Icon
// @namespace    wkfullscreen_icon
// @version      1.1.4
// @description  add a fullscreen icon to WaniKani for true fullscreen display on MacOS
// @include      /^https?://(www|preview)\.wanikani\.com/(review|lesson)/session$/
// @author       Théis Bazin (github.com/tbazin)
// @homepageURL  https://gist.github.com/tbazin/2383c1dd9f752623f79a5f4394af08db
// @downloadURL  https://gist.githubusercontent.com/tbazin/2383c1dd9f752623f79a5f4394af08db/raw/wanikani-fullscreen_icon.user.js
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    let container;
    let headerButtons = document.getElementById('header-buttons');
    if (headerButtons == null) {
        let summaryButtons = document.getElementById('summary-button');
        container = summaryButtons;
    }
    else {
        let buttons = headerButtons.getElementsByTagName('ul')[0];
        let fullscreenListItem = document.createElement('li');
        buttons.appendChild(fullscreenListItem);
        container = fullscreenListItem;
    }
    let fullscreenButton = document.createElement('a');
    fullscreenButton.id = 'fullscreen-mode';
    fullscreenButton.href = '#';
    container.appendChild(fullscreenButton);
    let fullscreenIcon = document.createElement('i');
    fullscreenIcon.id = 'fullscreen-icon';
    fullscreenIcon.classList.add('fa', 'fa-window-maximize');
    fullscreenButton.appendChild(fullscreenIcon);

    function focusResponseField(){
        let responseField = document.getElementById('user-response');
        responseField.focus();
    };

    function toggleFullscreen() {
        if (document.fullscreenElement) {
            // document.fullscreenElement is not null, the document is in fullscreen
            document.exitFullscreen()
        } else {
            document.addEventListener(
                'fullscreenchange',
                focusResponseField,
                {once: true});
            document.documentElement.requestFullscreen();
        };
    };

    fullscreenButton.addEventListener('click', toggleFullscreen);
    document.addEventListener('keydown', (keydownEvent) => {
        if (keydownEvent.ctrlKey && keydownEvent.keyCode == 70) {
            toggleFullscreen();
        }
    });
})();