mdb-ui-kit/scss/_menu.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

%menu-bottom-left {
  top: 100%;
  left: 0;
  transform-origin: 0 0;
}

%menu-bottom-right {
  right: 0;
  left: auto;
  transform-origin: 100% 0;
}

.dropdown-menu {
  display: block; // utilize transition transform instead of flipping display
  padding: .5rem 0;
  border: 0;
  opacity: 0;
  transition: transform $mdb-menu-expand-duration $mdb-animation-curve-default, opacity $mdb-menu-fade-duration $mdb-animation-curve-default;
  transform: scale(0);
  transform-origin: 0 0;
  will-change: transform;
  @include shadow-2dp();

  .open > & {
    //z-index: 999;
    opacity: 1;
    transform: scale(1);
  }

  // Default the alignment on the button to bottom right
  //@extend %menu-bottom-right;
  // FIXME: bootstrap alignment issue - https://github.com/twbs/bootstrap/issues/18852
  // FIXME: once the bug is fixed, want to default origin of menu open to the right of standard buttons

  &.dropdown-menu-left {
    @extend %menu-bottom-left;
  }

  &.dropdown-menu-right {
    @extend %menu-bottom-right;
  }

  &.dropdown-menu-top-left {
    top: auto;
    bottom: 0;  // needs to be overridden with the toggle-button height, see mdb-btn-icon above
    transform-origin: 0 100%;
  }

  &.dropdown-menu-top-right {
    top: auto;
    right: 0;
    bottom: 0;  // needs to be overridden with the toggle-button height, see mdb-btn-icon above
    left: auto;
    transform-origin: 100% 100%;
  }

  // https://www.google.com/design/spec/components/menus.html#menus-specs
  .dropdown-item {

    display: flex;

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

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

    padding-top: $mdb-menu-item-padding-top;
    padding-right: $mdb-menu-item-padding-right;
    padding-bottom: $mdb-menu-item-padding-bottom;
    padding-left: $mdb-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: $mdb-menu-line-height;
    text-overflow: ellipsis;
    word-wrap: break-word;


    // 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: $mdb-menu-item-padding-right-md;
      padding-left: $mdb-menu-item-padding-left-md;
    }
  }
}

.dropdown {

  .dropdown-toggle {

    &.mdb-btn-icon,
    &.mdb-btn-fab {
      // remove the dropdown icon
      &::after {
        display: none;
      }

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

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

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