pre {
    position: relative;
}

.copy-btn {
    position: absolute;
    top: 6px;
    right: 6px;
    padding: 2px 10px;
    background: #555;
    color: #fff;
    border: none;
    border-radius: 4px;
    cursor: pointer;
    font-size: 12px;
    opacity: 0;
    transition: opacity 0.2s;
    z-index: 10;
}

pre:hover .copy-btn {
    opacity: 1;
}