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

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

.sidebar-wrapper {
  display: flex;
  --d-sidebar-highlight-color: var(--primary-low);
  // 1.25rem gets text left-aligned with the hamburger icon
  --d-sidebar-row-horizontal-padding: 1.25rem;
  background-color: var(--primary-very-low);
  grid-area: sidebar;
  position: sticky;
  top: var(--header-offset);

  .footer-nav-ipad & {
    top: calc(var(--header-offset) + var(--footer-nav-height));
  }
  height: calc(100vh - var(--header-offset, 0));
  align-self: start;
  overflow-y: auto;

  .sidebar-row {
    padding: 0.33rem var(--d-sidebar-row-horizontal-padding);
    align-items: center;
    font-size: var(--font-down-1);
  }

  .sidebar-container {
    display: flex;
    flex-direction: column;
    box-sizing: border-box;
    height: 100%;
    width: 100%;
    padding: 0;
    overflow-x: hidden;
    // allows sidebar to scroll to the bottom when the composer is open
    height: calc(100% - var(--composer-height, 0));
  }

  .sidebar-sections {
    display: flex;
    flex-direction: column;
    box-sizing: border-box;
    flex: 1;
    padding: 1em 0;
    overflow-x: hidden;
    overflow-y: overlay;

    // custom scrollbar styling
    --scrollbarBg: transparent;
    --scrollbarThumbBg: var(--primary-low);
    --scrollbarWidth: 0.5em;

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

    &::-webkit-scrollbar {
      width: var(--scrollbarWidth);
    }

    &::-webkit-scrollbar-thumb {
      background-color: transparent;
      border-radius: calc(var(--scrollbarWidth) / 2);
    }

    &::-webkit-scrollbar-track {
      background-color: transparent;
    }

    &:hover {
      scrollbar-color: var(--scrollbarThumbBg) var(--scrollbarBg);

      &::-webkit-scrollbar-thumb {
        background-color: var(--scrollbarThumbBg);
      }

      transition-delay: 0s;
    }
  }
}

.sidebar-hamburger-dropdown {
  // note that these apply to mobile and desktop dropdowns
  .sidebar-section-wrapper {
    .sidebar-section-header-wrapper.sidebar-row {
      padding-right: 0;
    }

    .sidebar-section-header {
      padding: 0;
    }
  }

  .discourse-no-touch & {
    .sidebar-section-wrapper .sidebar-section-header-wrapper:hover,
    .sidebar-section-wrapper .sidebar-section-header-wrapper:focus-within {
      background: transparent;
    }
  }

  .sidebar-footer-wrapper {
    margin-top: 1em;
    .sidebar-footer-container {
      background: var(--secondary);
      &:before {
        background: linear-gradient(
          to bottom,
          transparent,
          rgba(var(--secondary-rgb), 1)
        );
      }
    }
  }
}