Changes WASD and shift-WASD to do the same as arrow keys.

fpuzzles-wasd.js

// ==UserScript==
// @name         Fpuzzles-WASD
// @namespace    http://tampermonkey.net/
// @version      1.3
// @description  Changes wasd to move between cells
// @author       Rangsk
// @match        https://*.f-puzzles.com/*
// @match        https://f-puzzles.com/*
// @icon         data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==
// @grant        none
// @run-at       document-end
// ==/UserScript==

(function() {
    const doShim = function() {
        'use strict';

        const prevOnKeyDown = document.onkeydown;
        document.onkeydown = function(event) {
            if (!event.key ||
                !['w', 'a', 's', 'd'].includes(event.key.toLowerCase()) ||
                disableInputs ||
                popup ||
                testPaused() ||
                toolCosmetics.includes(currentTool) ||
                event.ctrlKey ||
                event.altKey ||
                event.metaKey) {
                prevOnKeyDown(event);
                return;
            }
            event.preventDefault();

            if (selection.length) {
                var newCell;
                const key = event.key.toLowerCase();
                if (key === 'w') {
                    newCell = grid[(selection[selection.length - 1].i - 1 + size) % size][selection[selection.length - 1].j];
                } else if (key === 'd') {
                    newCell = grid[selection[selection.length - 1].i][(selection[selection.length - 1].j + 1 + size) % size];
                } else if (key === 's') {
                    newCell = grid[(selection[selection.length - 1].i + 1 + size) % size][selection[selection.length - 1].j];
                } else {
                    newCell = grid[selection[selection.length - 1].i][(selection[selection.length - 1].j - 1 + size) % size];
                }
                if (event.shiftKey) {
                    newCell.select();
                    selection.push(selection.splice(selection.indexOf(newCell), 1)[0]);
                } else {
                    selection = [newCell];
                }
            } else {
                selection.push(grid[0][0]);
            }
        }
    }

    let intervalId = setInterval(() => {
        if (!document.onkeydown) {
            return;
        }

        clearInterval(intervalId);
        doShim();
    }, 16);
})();