photo pea full width.md

What it basically does:

window.addEventListener('resize', _ => window.innerWidth = window.outerWidth + 300);

Just pasting it into the Developer Tools console does not update it, just jiggle the border to trigger the recalc.

PhotoPea full width.user.js

// ==UserScript==
// @name         PhotoPea full width
// @namespace    http://tampermonkey.net/
// @version      1.1
// @description  Use full width - move the ad outside the viewport
// @author       BananaAcid
// @match        https://www.photopea.com/
// @icon         https://www.google.com/s2/favicons?sz=64&domain=photopea.com
// @updateURL    https://gist.github.com/BananaAcid/920a0af587367101a95a30c546d0162d/raw/PhotoPea%2520full%2520width.user.js
// @downloadURL  https://gist.github.com/BananaAcid/920a0af587367101a95a30c546d0162d/raw/PhotoPea%2520full%2520width.user.js
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    function addGlobalStyle(css = '') {
        let target = (document.getElementsByTagName('head') || document.getElementsByTagName('body'))[0];
        let style = document.createElement('style');

        style.type = 'text/css';
        style.innerHTML = (css || arguments.length ? arguments[0][0] : '').replaceAll(';', ' !important;');
        target.appendChild(style);
    }

    let fn = _ => {window.innerWidth = window.outerWidth + 300};
    window.addEventListener('resize', fn); // worse because of app script being faster:  , _ => requestAnimationFrame(fn));

    fn(); // initial = on page load

    addGlobalStyle`
        .topbar > button.fitem.bbtn {
            position: relative;
            top: -3em;
        }
    `;

})();

To use or "install":

1. you need to have the Tempermonkey plugin installed in your browser
2. then click on the "Raw" button.