wagtail/client/src/components/Sidebar/menu/SubMenuItem.scss

.sidebar-sub-menu-trigger-icon {
  display: block;
  width: 20px;
  height: 20px;
  position: absolute;
  // Remove once we drop support for Safari 13.
  // stylelint-disable-next-line property-disallowed-list
  right: 15px;
  inset-inline-end: 15px;
  top: 0;
  bottom: 0;
  margin: auto 0;

  @include transition(
    transform $menu-transition-duration ease,
    width $menu-transition-duration ease,
    height $menu-transition-duration ease
  );

  &--open {
    transform-origin: 50% 50%;
    transform: rotate(180deg);
  }

  .sidebar--slim & {
    width: 16px;
    height: 16px;
    transform: translate3d(13px, 0, 0);
  }
}

.sidebar-sub-menu-panel {
  @apply w-flex w-flex-col w-bg-primary-200 w-h-screen;
  width: $menu-width;

  > h2,
  &__list {
    width: $menu-width;
  }

  > h2 {
    // w-min-h-[160px] and w-mt-[35px] classes are to vertically align the title and icon combination to the search input on the left
    @apply w-min-h-[160px] w-mt-[35px] w-text-white w-mb-0 w-inline-flex w-flex-col w-justify-center w-items-center;

    &:before {
      font-size: 4em;
      display: block;
      text-align: center;
      margin: 0 0 0.2em;
      width: 100%;
      opacity: 0.15;
    }
  }

  ul > li {
    position: relative;

    @include transition(border-color $menu-transition-duration ease);
  }

  > ul {
    flex-grow: 1;
    padding: 0;
    margin: 0;
    overflow-y: auto;
  }

  > ul > li {
    border: 0;
  }

  &__footer {
    margin: 0;
    padding: 0.9em 1.7em;
    text-align: center;
    color: $color-menu-text;
  }

  &--visible {
    visibility: visible;
    box-shadow: 2px 0 2px rgba(0, 0, 0, 0.35);
  }

  @at-root .sidebar--slim #{&} {
    transform: translate3d($menu-width-slim - $menu-width, 0, 0);
  }
  // Don't apply this to nested submenus though
  @at-root .sidebar--slim .sidebar-sub-menu-panel #{&} {
    transform: translate3d(0, 0, 0);
  }

  &--open {
    transform: translate3d($menu-width, 0, 0);

    // If another submenu is opening, display this menu behind it
    z-index: -1;

    @at-root .sidebar--slim #{&} {
      transform: translate3d($menu-width-slim, 0, 0);
    }
    // Don't apply this to nested submenus though
    @at-root .sidebar--slim .sidebar-sub-menu-panel #{&} {
      transform: translate3d($menu-width, 0, 0);
    }
  }
}

.sidebar-sub-menu-item {
  &--open {
    > a {
      text-shadow: -1px -1px 0 rgba(0, 0, 0, 0.3);
    }
  }
}