@mixin hairline($border-color: #eee) {
  content: '';
  position: absolute;
  top: -50%;
  left: -50%;
  right: -50%;
  bottom: -50%;
  transform: scale(0.5);
  pointer-events: none;
  box-sizing: border-box;
  transform-origin: center; /* cover wechat button:after default transforn-origin */
  border: 0 solid $border-color;
}