#screensaver {
    position: fixed;
    top: 0;
    left: 0;
    width: 100vw;
    height: 100vh;
    pointer-events: none;
    z-index: 9999;
    background: transparent;
    display: block;
    opacity: 0;
    transition: opacity 1s ease;
    /* フェード時間は調整可能 */
    pointer-events: none;
}

#screensaver.visible {
    opacity: 1;
    pointer-events: auto;
    /* 必要なら */
}

#screensaver.hidden {
    opacity: 0;
    pointer-events: none;
    /* display:none は使わず opacityのみで非表示 */
}