.dropdown-content {
    max-height: 0;            /* 高さ0で閉じた状態 */
    opacity: 0;               /* 完全透明 */
    overflow: hidden;
    transition: max-height 0.3s ease, opacity 0.3s ease;
    margin: 3px;
}

.dropdown-content.open {
    max-height: 500px;       /* 適当な最大高さ */
    opacity: 1;
}

.dropdown-toggle {
    cursor: pointer;
}