mdb-ui-kit/scss/_dropdown.scss

// Menus https://www.google.com/design/spec/components/menus.html#menus-specs
// Dropdown buttons (mobile and desktop) https://www.google.com/design/spec/components/buttons.html#buttons-dropdown-buttons

.dropdown-menu {
  display: none;
  padding: .25rem 0;
  border: 0;
  opacity: 0;
  transform: scale(0);
  transform-origin: 0 0;
  will-change: transform, opacity;
  transition: transform $bmd-menu-expand-duration $bmd-animation-curve-default,
              opacity $bmd-menu-fade-duration $bmd-animation-curve-default;

  @include media-breakpoint-up(md) {
    padding: .5rem 0;
  }

  &.showing {
    animation-name: bmd-dropdown-animation;
    animation-duration: $bmd-menu-expand-duration;
    animation-fill-mode: forwards;
    animation-timing-function: $bmd-animation-curve-default;
  }

  .open > &,
  &.show {
    display: block;
    opacity: 1;
    transform: scale(1);
  }

  &.hiding {
    display: block;
    opacity: 0;
    transform: scale(0);
  }

  &[x-placement="bottom-start"],
  &.dropdown-menu-left {
    transform-origin: 0 0;
  }

  &[x-placement="bottom-end"],
  &.dropdown-menu-right {
    transform-origin: 100% 0;
  }

  &[x-placement="top-start"] {
    transform-origin: 0 100%;
  }

  &[x-placement="top-end"] {
    transform-origin: 100% 100%;
  }

  // https://www.google.com/design/spec/components/menus.html#menus-specs
  .dropdown-item {
    // used to properly size the ripple container
    position: relative;

    display: flex;
    flex-flow: row wrap;
    align-items: center;

    min-width: $bmd-menu-item-min-width;
    max-width: $bmd-menu-item-max-width;
    min-height: $bmd-menu-item-min-height;

    padding: $bmd-menu-item-padding-top $bmd-menu-item-padding-right
      $bmd-menu-item-padding-bottom $bmd-menu-item-padding-left;

    // FIXME: multi-line menu word wrapping doesn't work - see the maximum width example in menus.md
    overflow: hidden;
    line-height: $bmd-menu-line-height;
    text-overflow: ellipsis;
    word-wrap: break-word;

    &.active,
    &:active {
      background-color: inherit;
      color: inherit;
    }

    // Simple menus always maintain a 16dp margin (phone) or 24dp margin (tablet) to the left and right edges of the screen.
    @include media-breakpoint-up(md) {
      padding-right: $bmd-menu-item-padding-right-md;
      padding-left: $bmd-menu-item-padding-left-md;
    }
  }
}

// this could be in a .btn-group or .dropdown
.dropdown-toggle {
  &.bmd-btn-icon,
  &.bmd-btn-fab {
    // remove the dropdown icon
    &::after {
      display: none;
    }

    ~ .dropdown-menu {
      &.dropdown-menu-top-left,
      &.dropdown-menu-top-right {
        bottom: $bmd-btn-icon-size; // push up the bottom of the menu the height of the button
      }
    }
  }

  &.bmd-btn-fab-sm {
    ~ .dropdown-menu {
      &.dropdown-menu-top-left,
      &.dropdown-menu-top-right {
        bottom: $bmd-btn-fab-size-sm; // push up the bottom of the menu the height of the button
      }
    }
  }

  &.bmd-btn-icon {
    ~ .dropdown-menu {
      // collapse some spacing
      margin: 0;
    }
  }

  .show &.btn {
    &,
    &-secondary,
    &-success,
    &-info,
    &-warning,
    &-danger {
      background-color: transparent;
    }
  }
}

@keyframes bmd-dropdown-animation {
  from {
    opacity: 0;
    transform: scale(0);
  }
  to {
    opacity: 1;
    transform: scale(1);
  }
}