MediaWiki:Citizen.js

function isUserLoggedIn() {
    return !mw.user.isAnon();
}

function handleElement($element) {
    if ($element.attr('aria-disabled') === 'true') {
        $element.after('<p>Please <a href="/wiki/Special:UserLogin">login</a> or <a href="/wiki/Special:UserSignup">sign up</a> to continue.</p>');
    }
}

function handleMutations(mutationsList) {
    mutationsList.forEach(function (mutation) {
        if (mutation.type === 'childList') {
            Array.prototype.forEach.call(mutation.addedNodes, function (node) {
                if ($(node).hasClass('oo-ui-buttonElement-button')) {
                    handleElement($(node));
                }
            });
        } else if (mutation.type === 'attributes' && mutation.attributeName === 'aria-disabled') {
            handleElement($(mutation.target));
        }
    });
}

var observer = new MutationObserver(handleMutations);
observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['aria-disabled'] });

$(document).ready(function () {
    $('.oo-ui-buttonElement-button').each(function (index, element) {
        handleElement($(element));
    });
});