This code provides a starting point for enhancing keyboard navigation accessibility within a megamenu. It adds arrow key navigation capabilities and enhances Escape and Tab key behavior to maintain proper focus when navigating via keyboard.

AccessibleKeyboardPatterns.js

/*
 *  All code is intended as learning examples only and will require modification to function properly based on the HTML structure of the menu itself.
 */

/**
 * Attaches a keyboard event listener to the given item and handles keyboard interactions.
 *
 * @param {HTMLElement} item - The menu item to attach the event listener to.
 * 
 */
function processKeyboardInput(item) {
    // Add event listener for keydown events when the menu item is focused.
    item.addEventListener('keydown', function (event) {
        // Prevent default behavior for arrow keys, ESC key, and tab.
        event.stopImmediatePropagation();
        // Setting up variables for the next and previous items in the menu.
        let nextListItem;
        let prevListItem;
        // Check if the current menu item in focus is the mobile menu toggle button.
        if (this.classList.contains('mobile-menu-toggle')) {
            // If the current item is the mobile menu toggle button, there are no previous sibling items to target.
            nextListItem = this.nextElementSibling;
            prevListItem = null;
        } else {
            // If the current item is a submenu toggle button, target the next and previous sibling items.
            nextListItem = this.closest('li').nextElementSibling;
            prevListItem = this.closest('li').previousElementSibling;
        }

        // Switch statement to handle different key events.
        switch (event.key) {
            // ESC key closes the current menu or submenu and returns focus to the parent button.
            case "Escape":
                // Setting up a target button variable that will be used to receive the focus.
                let targetButton;
                // If the current item is a mobile menu toggle button with an open mobile menu, set the target button value to the mobile menu toggle button.
                if (this.classList.contains('mobile-menu-toggle') && this.getAttribute('aria-expanded') == 'true') {
                    targetButton = this;
                } else if (this.classList.contains('megamenu-button')) {
                    // If the current item is a submenu toggle button with an open submenu, set the target button value to the submenu toggle button.
                    if (this.getAttribute('aria-expanded') == 'true') {
                        targetButton = this;
                    }
                    // If the current item is nested within an open submenu, set the target button value to the parent submenu toggle button.
                    else if (this.closest('.submenu-expanded')) {
                        targetButton = this.closest('.submenu-expanded');
                    }
                }
                // If the target button's menu is expanded, close the menu.
                if (targetButton.getAttribute('aria-expanded') == 'true') {
                    toggleMenu(targetButton);
                }
                // Focus on the target button.
                targetButton.focus();
                break;
            // Right arrow key moves focus to the next menu item moving from left to right, looping back to the first item in the menu if at the end.
            case "ArrowRight":
                // Prevent the default right arrow key behavior.
                event.preventDefault();
                // If the current item is a submenu toggle button and the submenu is open, close the open submenu.
                if (this.classList.contains('megamenu-button') && (this.getAttribute('aria-expanded') == 'true')) {
                    toggleMenu(this);
                }
                // If the current item is not nested within an open submenu, move focus to the next item.
                if (!this.closest('.submenu-expanded')) {
                    // If there is a next sibling item in the menu, move focus to it.
                    if (nextListItem) {
                        nextListItem.querySelector('a, button').focus();
                    } else {
                        // If this menu item is at the end of the menu, return focus to the first item in the menu.
                        this.closest('.megamenu').querySelector('li').querySelector('a, button').focus();
                    }
                } else {
                    // If the current item is nested within an open submenu, close the submenu and move focus to the next main-level menu item.
                    let openMenuButton = this.closest('.submenu-expanded').previousElementSibling;
                    // If there is an open submenu, close it.
                    if (openMenuButton) {
                        toggleMenu(openMenuButton);
                    }
                    // If there is a next top-level sibling item in the menu, move focus to it.
                    if (this.closest('.menu-item-level-0').nextElementSibling) {
                        this.closest('.menu-item-level-0').nextElementSibling.querySelector('a, button').focus();
                    } else {
                        // If this menu item is at the end of the top-level menu, return focus to the first top-level item in the menu.
                        this.closest('.megamenu').querySelector('li').querySelector('a, button').focus();
                    }
                }
                break;
            // Left arrow key moves focus to the previous menu item, looping back to the last item if at the beginning.
            case "ArrowLeft":
                // Prevent the default left arrow key behavior.
                event.preventDefault();
                // If the current item is a submenu toggle button and the submenu is open, close the open submenu.
                if (this.classList.contains('megamenu-button') && (this.getAttribute('aria-expanded') == 'true')) {
                    toggleMenu(this);
                }
                // If the current item is not nested within an open submenu, move focus to the previous item.
                if (!this.closest('.submenu-expanded')) {
                    // If there is a previous sibling item in the menu, move focus to it.
                    if (prevListItem) {
                        prevListItem.querySelector('a, button').focus();
                    } else {
                        // If this menu item is at the start of the menu, return focus to the last item in the menu.
                        this.closest('.megamenu').lastElementChild.querySelector('a, button').focus();
                    }
                } else {
                    // If the current item is nested within an open submenu, close the submenu and move focus to the previous main-level menu item.
                    let openMenuButton = this.closest('.submenu-expanded').previousElementSibling;
                    // If there is an open submenu, close it.
                    if (openMenuButton) {
                        toggleMenu(openMenuButton);
                    }
                    // If there is a previous top-level sibling item in the menu, move focus to it.
                    if (this.closest('.menu-item-level-0').previousElementSibling) {
                        this.closest('.menu-item-level-0').previousElementSibling.querySelector('a, button').focus();
                    } else {
                        // If this menu item is at the start of the top-level menu, return focus to the last top-level item in the menu.
                        this.closest('.megamenu').lastElementChild.querySelector('a, button').focus();
                    }
                }
                break;
            // Up arrow key moves focus to the previous menu item if in a submenu, looping back to the last item if at the beginning.
            case "ArrowUp":
                // Prevent the default up arrow key behavior.
                event.preventDefault();
                // If the current item is within a submenu dropdown and there is a previous sibling item, move focus to it.
                if (this.closest('.submenu-expanded') && prevListItem) {
                    prevListItem.querySelector('a, button').focus();
                } else if(this.closest('.submenu-expanded') && prevListItem) {
                    // If the current item is the first item within a submenu dropdown, move focus to the last item in the submenu.
                    this.closest('.submenu-expanded').lastElementChild.querySelector('a, button').focus();
                }
                break;
            // Down arrow key opens submenus and mobile menus, focusing on the first menu item. 
            // If in a submenu, down arrow key moves focus to the next menu item, looping back to the first item if at the end.
            case "ArrowDown":
                // Prevent the default down arrow key behavior.
                event.preventDefault();
                // If the current item is the mobile menu toggle button, open the menu and focus on the first item.
                if (this.classList.contains('mobile-menu-toggle')) {
                    // Check that the mobile menu is not already open before toggling.
                    if (this.getAttribute('aria-expanded') == 'false') {
                        toggleMenu(this);
                    }
                    // Focus on the first item in the mobile menu.
                    this.nextElementSibling.querySelector('li').querySelector('a, button').focus();
                } else if (this.classList.contains('megamenu-button')) {
                    // If the current item is a submenu toggle button, open the menu and focus on the first item.
                    if (this.getAttribute('aria-expanded') == 'false') {
                        toggleMenu(this);
                    }
                    this.nextElementSibling.querySelector('li').querySelector('a, button').focus();
                } else {
                    // If the current item is within a submenu dropdown and there is a next sibling item, move focus to it.
                    if (this.closest('.submenu-expanded') && nextListItem) {
                        nextListItem.querySelector('a, button').focus();
                    } else if (this.closest('.submenu-expanded') && !nextListItem) {
                        // If the current item is the last item within a submenu dropdown, move focus to the first item in the submenu.
                        this.closest('.submenu-expanded').querySelector('li:first-child').querySelector('a, button').focus();
                    }
                }
                break;
            // Tab key closes the currently open submenu and moves focus to the next menu item.
            case "Tab":
                // If reverse-tabbing out of a submenu, close the submenu and move focus to the parent menu item.
                if (event.shiftKey) {
                    if (!prevListItem && this.closest('li').classList.contains('menu-item-level-1')) {
                        toggleMenu(this.closest('.menu-item-level-0').querySelector('a, button'));
                    }
                } else if (!nextListItem && !event.shiftKey && !this.classList.contains('megamenu-button')) {
                    // If tabbing past the last item of a submenu, close the submenu and move focus to the next main-level menu item.
                    toggleMenu(this.closest('.menu-item-level-0').querySelector('a, button'));
                }
                break;
        }
    });
}

/**
 * Toggles the menu open or closed based on the provided trigger button.
 * @param {HTMLElement} triggerButton - The button element that triggers the menu toggle.
 */
function toggleMenu(triggerButton) {
    // If the trigger button is the mobile menu toggle button, toggle the mobile menu.
    if (triggerButton.classList.contains('mobile-menu-toggle')) {
        // If the mobile menu is already expanded, close it.
        if (triggerButton.getAttribute('aria-expanded') == 'true') {
            triggerButton.nextElementSibling.classList.remove('menu-expanded');
            triggerButton.setAttribute('aria-expanded', 'false');
        } else {
            // If the mobile menu is closed, expand it.
            triggerButton.nextElementSibling.classList.add('menu-expanded');
            triggerButton.setAttribute('aria-expanded', 'true');
        }
    } else {
        // Submenu toggle actions.
        var targetSubMenu = triggerButton.nextElementSibling;
        // If the submenu is already expanded, close it.
        if (triggerButton.getAttribute('aria-expanded') == 'true') {
            triggerButton.setAttribute('aria-expanded', 'false');
            targetSubMenu.classList.remove('submenu-expanded');
        } else {
            // If the submenu is closed, close any other open submenus and expand the target submenu.
            if (!triggerButton.closest('.submenu-expanded')) {
                // Target all submenu buttons and close their submenus.
                document.querySelectorAll('.megamenu-button').forEach(function (button) {
                    button.setAttribute('aria-expanded', 'false');
                    button.parentElement.querySelectorAll('.sub-menu').forEach(function (sibling) {
                        sibling.classList.remove('submenu-expanded');
                    });
                });
            }
            // Expand the target submenu.
            triggerButton.setAttribute('aria-expanded', 'true');
            targetSubMenu.classList.add('submenu-expanded');
        }
    }
}