Virtualize the Fullscreen API to expand elements without using native fullscreen. This is useful when using a window manager that doesn't like it when windows try to go fullscreen.

fullscreen-api-virtualizer.user.js

// ==UserScript==
// @name         Fullscreen API Virtualizer
// @namespace    https://agartner.com
// @version      1.0
// @description  Virtualize the Fullscreen API to expand elements without using native fullscreen
// @author       You
// @match        *://*/*
// @grant        none
// @run-at       document-start
// ==/UserScript==

(function() {
    'use strict';

    // Add CSS for virtualized fullscreen
    document.head.appendChild(Object.assign(document.createElement('style'), {
        textContent: `
            .virtualized-fullscreen {
                position: fixed !important;
                top: 0 !important;
                left: 0 !important;
                width: 100vw !important;
                height: 100vh !important;
                z-index: 2147483647 !important;
                background-color: black !important;
                margin: 0 !important;
                padding: 0 !important;
                border: none !important;
                box-sizing: border-box !important;
            }
        `
    }));

    // Track fullscreen state
    const fullscreenState = {
        active: false,
        element: null,
        originalStyles: null
    };

    // Style management functions
    function saveOriginalStyles(element) {
        const stylesToSave = ['position', 'top', 'left', 'width', 'height', 'zIndex',
                            'backgroundColor', 'margin', 'padding', 'border', 'boxSizing'];
        const originalStyles = {};

        stylesToSave.forEach(prop => {
            originalStyles[prop] = element.style[prop];
        });

        return originalStyles;
    }

    function restoreOriginalStyles(element, styles) {
        Object.keys(styles).forEach(prop => {
            element.style[prop] = styles[prop];
        });
    }

    // Virtual fullscreen implementation
    function virtualRequestFullscreen() {
        if (fullscreenState.active) return Promise.resolve();

        const element = this;
        fullscreenState.element = element;
        fullscreenState.originalStyles = saveOriginalStyles(element);
        fullscreenState.active = true;

        element.classList.add('virtualized-fullscreen');

        document.dispatchEvent(new Event('fullscreenchange'));
        return Promise.resolve();
    }

    function virtualExitFullscreen() {
        if (!fullscreenState.active) return Promise.resolve();

        const element = fullscreenState.element;
        element.classList.remove('virtualized-fullscreen');
        restoreOriginalStyles(element, fullscreenState.originalStyles);

        fullscreenState.active = false;
        fullscreenState.element = null;
        fullscreenState.originalStyles = null;

        document.dispatchEvent(new Event('fullscreenchange'));
        return Promise.resolve();
    }

    // Helper function to override methods safely
    function overrideMethod(object, methodName, newImplementation) {
        if (typeof object[methodName] === 'function') {
            object[methodName] = newImplementation;
        }
    }

    // Helper function to override getters safely
    function overrideGetter(object, propertyName, getter) {
        if (Object.getOwnPropertyDescriptor(object, propertyName)) {
            Object.defineProperty(object, propertyName, { get: getter });
        }
    }

    // Override request methods
    const requestMethods = ['requestFullscreen', 'mozRequestFullScreen',
                          'webkitRequestFullscreen', 'msRequestFullscreen'];

    requestMethods.forEach(method => {
        overrideMethod(Element.prototype, method, virtualRequestFullscreen);
    });

    // Override exit methods
    const exitMethods = ['exitFullscreen', 'mozCancelFullScreen',
                        'webkitExitFullscreen', 'msExitFullscreen'];

    exitMethods.forEach(method => {
        overrideMethod(Document.prototype, method, virtualExitFullscreen);
    });

    // Override element getters
    const elementGetters = ['fullscreenElement', 'webkitFullscreenElement',
                          'mozFullScreenElement', 'msFullscreenElement'];

    elementGetters.forEach(getter => {
        overrideGetter(Document.prototype, getter, function() {
            return fullscreenState.active ? fullscreenState.element : null;
        });
    });

    // Override enabled getters
    const enabledGetters = ['fullscreenEnabled', 'webkitFullscreenEnabled',
                          'mozFullScreenEnabled', 'msFullscreenEnabled'];

    enabledGetters.forEach(getter => {
        overrideGetter(Document.prototype, getter, function() {
            return true;
        });
    });

    // Add Escape key handler
    document.addEventListener('keydown', function(event) {
        if (event.key === 'Escape' && fullscreenState.active) {
            virtualExitFullscreen();
        }
    });

    console.log('Fullscreen API Virtualizer loaded successfully!');
})();