.submenu-toggle::after {
     content: "▶";
     position: absolute;
     right: 12px;
     top: 50%;
     transform: translateY(-50%);
     pointer-events: none;
     /* ⭐ QUAN TRỌNG */
     transition: transform .2s ease;
     font-size: 10px;
 }
@media (max-width: 991px) {

    .submenu>.dropdown-menu {
        display: none;
        margin-left: 0;
    }

    .submenu.show>.dropdown-menu {
        display: block;
    }

    .submenu-toggle {
        position: relative;
        padding-right: 32px;
        /* chừa chỗ cho mũi tên */
    }

    .submenu-toggle::after {
        content: "▶";
        position: absolute;
        right: 12px;
        top: 50%;
        transform: translateY(-50%);
        pointer-events: none;
        /* ⭐ QUAN TRỌNG */
        transition: transform .2s ease;
    }

    .submenu.show>.submenu-toggle::after {
        transform: translateY(-50%) rotate(90deg);
    }
}