@import "../../vari";

.up-mask {
  z-index: 100; //less then ui:z-index:1104
  &:after {
    content: '';
    position: fixed;
    z-index: 99;
    background-color: @default-mask-color;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
  }
}

目前的解决：增加一个 相邻 dom

@import "../../vari";

.up-mask {
  z-index: 100; //less then ui:z-index:1104
  //&:after {
  //  content: '';
  //  position: fixed;
  //  z-index: 99;
  //  background-color: @default-mask-color;
  //  top: 0;
  //  right: 0;
  //  bottom: 0;
  //  left: 0;
  //}
  // after & before 会遮挡元素本身
  & + .mask {
    content: '';
    position: fixed;
    z-index: 99;
    background-color: @default-mask-color;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
  }
}

after & before 感觉还是对应dom的内嵌子dom；导致没法设置低于自身的z-index。也许是我的写法不对？