css/modules/primer-avatars/lib/avatar-stack.scss

// Stacked avatars can be used to show who is participating in thread when
// there is limited space available.
//
// No styleguide references
.avatar-stack {
  display: inline-block;
  white-space: nowrap;

  .avatar {
    position: relative;
    z-index: 2;
    display: inline-block;
    width: 20px;
    height: 20px;
    box-sizing: content-box;
    margin-right: -15px;
    background-color: $bg-white;
    border-right: 1px solid $white;
    border-radius: 2px;
    transition: margin 0.1s ease-in-out;

    &:only-child {
      background-color: transparent;
    }

    &:first-child {
      z-index: 3;
    }

    &:last-child {
      z-index: 1;
      margin-right: 0;
      border-right: 0;
    }
  }

  // When more than one avatar, margin-left will animate from -15 to 2, and reveal the stack
  &:hover .avatar {
    margin-right: 3px;

    &:last-child {
      margin-right: 0;
    }
  }
}

// Refactored, new avatar stack:

.AvatarStack {
  position: relative;
  min-width: 26px;
  height: 20px;
}

.AvatarStack-body {
  background: inherit;

  .avatar {
    position: relative;
    z-index: 2;
    display: inline-block;
    width: 20px;
    height: 20px;
    box-sizing: content-box;
    margin-right: -15px;
    background-color: $bg-white;
    border-right: $border-width $border-style $white;
    border-radius: 2px;
    transition: margin 0.1s ease-in-out;

    &:first-child {
      z-index: 3;
    }

    &:last-child {
      z-index: 1;
      margin-right: 0;
      border-right: 0;
    }

    // stylelint-disable selector-max-type
    img {
      border-radius: 2px;
    }
    // stylelint-enable selector-max-type

    // Account for 4+ avatars
    &:nth-child(n+4) {
      display: none;
      opacity: 0;
    }
  }

  &:hover .avatar:nth-child(n+4) {
    display: inline-block;
    opacity: 1;
  }

  &:hover .avatar {
    margin-right: 3px;
  }
}

.avatar.avatar-more {
  z-index: 1;
  margin-right: 0;
  background: $gray-100;

  &::before,
  &::after {
    position: absolute;
    display: block;
    height: 20px;
    content: "";
    border-radius: 2px;
    outline: $border-width $border-style $white;
  }

  &::before {
    z-index: 2;
    width: 14px;
    background: $gray-300;
  }

  &::after {
    width: 17px;
    background: $gray-200;
  }
}

.AvatarStack-body:hover .avatar-more {
  display: none;
}

// Temp tooltip class to position it correctly

.temp-tooltipped-align-left {
  &::after {
    left: 0 !important;
    margin-left: 0 !important;
  }

  &::before {
    left: 5px !important;
  }
}