.appBack {
    opacity: 0;
    display: none;

    background: none;
    border: none;
    outline: none;
    -webkit-appearance: none;
    -moz-appearance: none;
    appearance: none;
    transition: none;

    position: fixed;
    bottom: 12px;
    left: 12px;
    z-index: 10000;
    background-color: rgba(0,0,0,.8);
    width: 50px;
    height: 50px;
    border-radius: 50%;
    color: #fff;

    /* displayはflutter側で制御 */
    /* display: flex; */
    align-items: center;
    justify-content: center;
    font-size: 20px;
}
.appBack:active {
    opacity: .5;
}