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;
}

#main-outlet-wrapper {
  .sidebar-wrapper {
    --d-sidebar-highlight-color: var(--primary-low);

    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);
  }

  .sidebar-container {
    display: flex;
    flex-direction: column;
    box-sizing: border-box;
    height: 100%;
    width: var(--d-sidebar-width);
    padding: 1em 0 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;
    scroll-behavior: smooth;

    &::-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));
    box-sizing: border-box;
    flex: 1;
    display: flex;
    flex-direction: column;

    .composer-open & {
      // allows sidebar to scroll to the bottom when the composer is open
      margin-bottom: var(--composer-height);
      padding-bottom: var(--composer-ipad-padding);
    }
  }

  .sidebar-sections {
    flex: 1;
  }
}