js固定定位

<button id="backToTopBtn">返回顶部</button>

#backToTopBtn {
    position: fixed;
    bottom: 20px;
    right: 30px;
    z-index: 99;
    font-size: 18px;
    border: none;
    outline: none;
    background-color: #555;
    color: white;
    cursor: pointer;
    padding: 15px;
    border-radius: 4px;
}

#backToTopBtn:hover {
    background-color: #777;
}

document.getElementById('backToTopBtn').addEventListener('click', function() {
    window.scrollTo({ top: 0, behavior: 'smooth' });
});

.element1 {
    position: fixed;
    z-index: 100;
}

.element2 {
    position: fixed;
    z-index: 200; /* 确保这个元素在element1之上 */
}

@media (max-width: 600px) {
    #backToTopBtn {
        bottom: 10px;
        right: 10px;
    }
}