discourse/app/assets/stylesheets/common/base/sidebar.scss

:root {
  --d-sidebar-width: 16em;
  --d-sidebar-animation-time: 0.25s;
  --d-sidebar-animation-ease: ease-in-out;
}

.header-sidebar-toggle {
  --toggle-padding: 0.5em;
  margin-right: 0.75em;
  // extending the toggle beyond the page when space allows
  // for better logo alignment with content
  @media screen and (min-width: 1380px) {
    margin-left: -3.5em;
  }

  // align on icon, because button is transparent
  @media screen and (max-width: 1480px) {
    margin-left: calc(var(--toggle-padding) * -1.3);
  }

  // prevents toggle overflow on smaller screens
  @media screen and (max-width: 1379px) {
    :not(.mobile-view) .has-sidebar-page & {
      margin-left: initial;
    }
  }

  transition: margin var(--d-sidebar-animation-speed)
    var(--d-sidebar-animation-ease);

  button {
    position: relative;
    font-size: var(--font-up-2);
    padding: var(--toggle-padding);

    .discourse-no-touch & {
      &:hover {
        background: var(--primary-low);
        .d-icon {
          color: var(--primary-medium);
        }
      }
    }
  }
}

#main-outlet-wrapper {
  .sidebar-wrapper {
    grid-area: sidebar;
    position: sticky;
    top: var(--header-offset);
    height: calc(100vh - var(--header-offset));
    align-self: start;
    overflow-y: auto;
    background-color: var(--primary-very-low);
    .discourse-touch &,
    &:hover {
      .sidebar-section-header-caret {
        opacity: 1;
        transition-delay: 0s;
      }
    }
  }

  .sidebar-container {
    box-sizing: border-box;
    height: 100%;
    width: var(--d-sidebar-width);
    padding: 1em 0;
    overflow-x: hidden;
    overflow-y: auto;

    // custom scrollbar styling
    --scrollbarBg: transparent;
    --scrollbarThumbBg: var(--primary-low);
    --scrollbarWidth: 1.2em;

    scrollbar-color: transparent var(--scrollbarBg);
    transition: scrollbar-color 0.25s ease-in-out;
    transition-delay: 0.5s;

    &::-webkit-scrollbar-thumb {
      background-color: transparent;
      border-radius: calc(var(--scrollbarWidth) / 2);
      border: calc(var(--scrollbarWidth) / 4) solid var(--primary-very-low);
    }
    &:hover {
      scrollbar-color: var(--scrollbarThumbBg) var(--scrollbarBg);
      &::-webkit-scrollbar-thumb {
        background-color: var(--scrollbarThumbBg);
      }
      transition-delay: 0s;
    }
    &::-webkit-scrollbar {
      width: var(--scrollbarWidth);
    }
  }

  .sidebar-scroll-wrap {
    // limit the wrapper width, so when the scrollbar is added the content doesn't shift
    max-width: calc(var(--d-sidebar-width) - var(--scrollbarWidth));
  }

  .sidebar-toggle {
    display: flex;
    justify-content: flex-end;
  }

  .sidebar-section-wrapper {
    margin-bottom: 1em;
  }

  .sidebar-section-header {
    display: flex;
    text-transform: uppercase;
    font-size: 1em;
    font-weight: bold;
    align-items: stretch;
  }

  .sidebar-section-header-link {
    @include ellipsis;
    flex: 1 1 auto;
    color: var(--primary);
    font-size: var(--font-down-1);
    padding: 0.25em 0.5em;

    &:visited {
      color: var(--primary);
    }

    &:hover {
      background: var(--primary-low);
    }
  }

  .sidebar-section-header-button {
    background: none;
    border: none;
    padding: 0.25em 0.5em;

    .d-icon {
      font-size: $font-down-1;
      color: var(--primary-medium);
    }

    &:hover {
      background: var(--primary-low);
    }
  }

  .sidebar-section-link-wrapper {
    margin-left: 1.5em;
  }

  .sidebar-section-link {
    display: flex;
    align-items: center;
    padding: 0.35em 0.5em;
    color: var(--primary-high);
    font-size: var(--font-down-1);
    transition: background-color 0.25s;

    &:hover {
      background: var(--primary-low);
    }

    &.active {
      color: var(--primary);
      font-weight: bold;
    }

    .sidebar-section-link-content-text {
      @include ellipsis;

      .badge-wrapper {
        font-size: 100%;
        width: 100%;

        .category-name {
          white-space: nowrap;
          overflow: hidden;
          text-overflow: ellipsis;
        }
      }
    }
  }

  .sidebar-section-message-wrapper {
    display: flex;
    margin-left: 1.5em;
  }

  .sidebar-section-message {
    padding: 0.25em 0.5em;
    font-size: var(--font-down-1);
    color: var(--primary-high);
  }

  .sidebar-section-link-content-badge {
    font-size: var(--font-down-1);
    color: var(--tertiary);
    margin-left: auto;
    font-weight: normal;
    margin-right: 0.4em;
  }

  .sidebar-section-header-caret {
    flex: 0 0 auto;
    width: 1.5em;
    display: flex;
    justify-content: flex-end;
    align-items: center;
    border: none;
    background: transparent;
    padding: 0;
    opacity: 0;
    transition: opacity 0.25s;
    transition-delay: 0.5s;
    .discourse-no-touch & {
      &:hover {
        svg {
          color: var(--primary-high);
        }
      }
    }

    svg {
      display: block;
      flex: 0 0 auto;
      transition: color 0.25s;
    }

    .d-icon {
      font-size: $font-down-1;
      color: var(--primary-medium);
    }
  }

  .sidebar-section-content {
    hr {
      margin: 0em 1.5em;
    }
  }

  .sidebar-section-link-personal-messages-sent,
  .sidebar-section-link-personal-messages-new,
  .sidebar-section-link-personal-messages-archive,
  .sidebar-section-link-personal-messages-unread,
  .sidebar-section-link-group-messages-new,
  .sidebar-section-link-group-messages-unread,
  .sidebar-section-link-group-messages-archive {
    margin-left: 0.5em;
  }
}