.dropdown-submenu {
    position:relative;
}
.dropdown-submenu>.dropdown-menu {
    top:0;
    left:-10rem; /* 10rem is the min-width of dropdown-menu */
    margin-top:-6px;
}

/* rotate caret on hover */
.dropdown-menu > li > a:hover:after {
    text-decoration: underline;
    transform: rotate(-90deg);
}