#bttbutton{
    position: fixed;
    right: 24px;
    bottom: 36px;                 /* aligné avec le bouton Ally */
    z-index: 9990;
    width: 44px; height: 44px;
    display: flex; align-items: center; justify-content: center;
    padding: 0;
    background-color: #0141DA,;    /* contraste 6:1 avec le texte blanc */
    color: #ffffff;
    border: none; border-radius: 50%;
    cursor: pointer;
    box-shadow: 0 2px 8px rgba(0,0,0,.25);
    opacity: 0;
    visibility: hidden;           /* sort le bouton de l'ordre de tabulation quand il est masqué */
    transform: translateY(10px);
    transition: opacity .25s ease, transform .25s ease, visibility .25s, background-color .2s ease;
}
#bttbutton.is-visible{ opacity: 1; visibility: visible; transform: translateY(0); }
#bttbutton:hover{ background-color: #0132a8; }        /* plus foncé, contraste 7,5:1 */
#bttbutton:focus-visible{ outline: 5px solid #ffffff; outline-offset: 5px; background-color: #0132a8; }
#bttbutton svg{ width: 28px; height: 28px; flex: 0 0 auto; }

@media (prefers-reduced-motion: reduce){
    #bttbutton transition: opacity .01ms, visibility .01ms; transform: none; }
}