animation-fill-mode: forwards 导致z-index失效

Question

遮罩z-index 1040
弹窗z-index 1050
#content 绝对定位 没设层级 #content弹窗是父子关系 遮罩和#content是兄弟关系

#content {
    -webkit-animation: fadeIn .4s;
    animation: fadeIn .4s;
    -webkit-animation-fill-mode: forwards;
    animation-fill-mode: forwards;
    /* -webkit-transition: opacity .4s;
    -o-transition: opacity .4s;
    transition: opacity .4s;
    opacity: 0; */
}

#content.fade-in {
    opacity: 1;
}

@-webkit-keyframes fadeIn {
    0% {
        opacity: 0;
    }

    to {
        opacity: 1;
    }
}

@keyframes fadeIn {
    0% {
        opacity: 0;
    }

    to {
        opacity: 1;
    }
}

Answer 1

现在的解决方案是 让 弹窗.modal 容器#content 和遮罩 都是兄弟节点 即可解决 但是为什么会出现这种问题 不清楚