picocss/scss/components/_dropdown.scss

details[role="list"] {
  position: relative;
  padding: 0;
  border-bottom: none;

  summary {
    margin: 0;

    & + ul {
      display: flex;
      z-index: 99;
      position: absolute;
      top: auto;
      right: 0;
      left: 0;
      flex-direction: column;
      min-width: fit-content;
      margin: 0;
      padding: 0;
      border: var(--border-width) solid var(--dropdown-border-color);
      border-radius: var(--border-radius);
      border-top-right-radius: 0;
      border-top-left-radius: 0;
      background-color: var(--dropdown-background-color);
      box-shadow: var(--card-box-shadow);
      color: var(--dropdown-color);
      white-space: nowrap;

      li {
        width: 100%;
        margin-bottom: 0;
        padding: calc(var(--form-element-spacing-vertical) * 0.5)
          var(--form-element-spacing-horizontal);
        list-style: none;

        &:first-of-type {
          margin-top: calc(var(--form-element-spacing-vertical) * 0.5);
        }

        &:last-of-type {
          margin-bottom: calc(var(--form-element-spacing-vertical) * 0.5);
        }

        a {
          display: block;
          margin: calc(var(--form-element-spacing-vertical) * -0.5)
            calc(var(--form-element-spacing-horizontal) * -1);
          padding: calc(var(--form-element-spacing-vertical) * 0.5)
            var(--form-element-spacing-horizontal);
          color: var(--dropdown-color);
          text-decoration: none;

          &:hover {
            background-color: var(--dropdown-hover-background-color);
          }
        }
      }
    }

    // Marker
    &::after {
      height: calc(1rem * var(--line-height));
      transform: rotate(0deg);
    }
  }

  &[disabled] summary,
  &.disabled summary {
    border-color: var(--form-element-disabled-border-color);
    background-color: var(--form-element-disabled-background-color);
    opacity: var(--form-element-disabled-opacity);
    pointer-events: none;
  }

  &[open] summary {
    border-bottom-right-radius: 0;
    border-bottom-left-radius: 0;
    &::before {
      display: block;
      z-index: 1;
      position: fixed;
      top: 0;
      right: 0;
      bottom: 0;
      left: 0;
      background: transparent;
      content: "";
      cursor: default;
    }
  }
}

// <summary> as a <select>
*:not(nav > ul > li):not(nav > aside > ul > li) {
  > details[role="list"] {
    summary {
      &:not([role="button"]) {
        height: calc(
          1rem * var(--line-height) + var(--form-element-spacing-vertical) * 2 +
            var(--border-width) * 2
        );
        padding: var(--form-element-spacing-vertical)
          var(--form-element-spacing-horizontal);
        border: var(--border-width) solid var(--form-element-border-color);
        border-radius: var(--border-radius);
        background-color: var(--form-element-background-color);
        color: var(--form-element-placeholder-color);
        line-height: inherit;
        cursor: pointer;

        @if $enable-transitions {
          transition: background-color var(--transition),
            border-color var(--transition), color var(--transition),
            box-shadow var(--transition);
        }

        &:active,
        &:focus {
          border-color: var(--form-element-active-border-color);
          background-color: var(--form-element-active-background-color);
        }

        &:focus {
          box-shadow: 0 0 0 var(--outline-width) var(--form-element-focus-color);
        }
      }
    }
  }
}

// Dropdown inside <nav>
nav ul li {
  details[role="list"] {
    display: inline-block;
    margin: calc(var(--nav-link-spacing-vertical) * -1)
      calc(var(--nav-link-spacing-horizontal) * -1);

    summary:not([role="button"]) {
      padding: var(--nav-link-spacing-vertical)
        var(--nav-link-spacing-horizontal);
      border-radius: var(--border-radius);
      line-height: var(--line-height);

      &:focus {
        background-color: var(--primary-focus);
      }

      a {
        pointer-events: none;

        &:focus {
          background-color: transparent;
        }
      }
    }

    &[open] summary {
      border-radius: var(--border-radius);

      & + ul {
        margin-top: calc(var(--nav-link-spacing-horizontal) * 0.5);
        border-radius: var(--border-radius);

        a {
          border-radius: 0;
        }
      }

      &[role="button"] {
        & + ul {
          margin-top: var(--nav-link-spacing-horizontal);
        }
      }
    }
  }
}