devdocs/assets/stylesheets/components/_sidebar.scss

//
// Sidebar
//

._sidebar {
  position: absolute;
  z-index: var(--sidebarZ);
  top: 0;
  bottom: 0;
  left: 0;
  overflow-x: hidden;
  overflow-y: scroll;
  padding-top: var(--headerHeight);
  background: var(--sidebarBackground);
  background-clip: content-box;
  -webkit-overflow-scrolling: touch;
  -ms-overflow-style: none; // IE 10 doesn't support pointer-events
  @extend %border-box;
  @extend %user-select-none;

  &:focus { outline: none; }

  ._overlay-scrollbars & {
    padding-top: 0;
    top: var(--headerHeight);
  }

  body:not(._native-scrollbars) & {
    &::-webkit-scrollbar { width: 10px; }
    &::-webkit-scrollbar-track {
      background: var(--contentBackground);
      border: 0;
    }
    &::-webkit-scrollbar-thumb {
      border-width: 3px;
      &:hover, &:active { border-width: 2px; }
    }
  }

  ._sidebar-hidden & {
    transform: translateX(-95%);
    transform: translateX(calc(.5rem - 100%));
    opacity: 0;
  }

  &:hover:not(.no-hover),
  &.show {
    transform: none;
    opacity: 1;
  }
}

._resizer {
  position: absolute;
  z-index: var(--headerZ);
  top: var(--headerHeight);
  bottom: var(--headerHeight);
  left: var(--sidebarWidth);
  margin-left: -2px;
  width: 3px;
  cursor: col-resize;

  ._sidebar-hidden & { display: none; }
  ._sidebar-hidden ._sidebar.show ~ & { display: block; }
}

//
// List
//

._list {
  margin: 0;
  padding: 0;
  list-style: none;
  width: var(--sidebarWidth);
  box-shadow: inset -1px 0 var(--sidebarBorder);
  @extend %border-box;

  @include mobile { width: var(--sidebarMediumWidth); }

  ._sidebar > & { min-height: 100%; }

  a:focus { outline: 0; }
}

._list-title {
  position: relative;
  margin: .5rem 0 0;
  padding: 0 .75rem 0 2.125rem;
  line-height: 2rem;
  font-size: .75rem;
  color: var(--textColorLight);
  text-transform: uppercase;
  cursor: default;
}

._list-title-link {
  display: none;
  float: right;
  font-weight: normal;
  text-transform: none;

  ._list-title:hover > & { display: block; }
}

._list-item {
  display: block;
  position: relative;
  padding: .25rem .75rem;
  line-height: 1.5rem;
  font-size: .875rem;
  cursor: default;
  background: var(--sidebarBackground);
  box-shadow: inset -1px 0 var(--sidebarBorder);
  @extend %truncate-text;

  &, &:hover {
    color: inherit;
    text-decoration: none;
  }

  &.focus,
  &.focus:hover,
  &.active,
  &.active:hover {
    color: var(--focusText);
    background: var(--focusBackground);
    box-shadow: inset -1px 0 var(--focusBorder);
  }

  &.active,
  &.active:hover {
    color: var(--selectionText);
    background: var(--selectionBackground);
    box-shadow: inset -1px 0 var(--selectionBorder);
  }

  &:before {
    float: left;
    margin: .25rem .625rem 0 0;
    @extend %doc-icon;
  }
}

._list-text {
  display: block;
  pointer-events: none;
  @extend %truncate-text;
}

._list-count, ._list-enable {
  float: right;
  font-size: .75rem;
  margin-left: .375rem;

  .focus > &,
  .active > & {
    color: inherit;
  }
}

._list-count {
  color: var(--textColorLighter);
  pointer-events: none;

  ._list-disabled:hover > & { display: none; }
}

._list-enable {
  display: none;
  color: var(--linkColor);
  cursor: pointer;

  &:hover { text-decoration: underline; }
  ._list-disabled:hover > &, ._list-result > & { display: block; }
  ._list-result.active > & { margin-right: -1rem; }
}

//
// List hierarchy
//

._list-dir:not(._list-rdir),
%_list-dir {
  padding-left: 2.125rem;
}

._list-disabled {
  @extend %_list-dir;

  &, &:hover { color: var(--textColorLight); }
  &:before { opacity: .7; }
}

._list-arrow {
  position: absolute;
  top: 0;
  left: .25rem;
  padding: .5rem .375rem .5rem .5rem;
  width: 1rem;
  height: 1rem;
  cursor: pointer;
  fill: var(--absolute);
  opacity: .4;

  &:hover { opacity: .65; }

  ._list-rdir > & {
    left: auto;
    right: .25rem;
  }

  .open > &, .open-title > & {
     -webkit-transform: rotate(90deg);
             transform: rotate(90deg);
   }
}

._list-sub {
  display: none;

  .open + & { display: block; }
  > ._list-item { padding-left: 2.375rem; }
  > ._list-dir, > ._list-sub > ._list-item { padding-left: 2.75rem; }
  > ._list-disabled { padding-left: 3.75rem; }
  > ._list-item:before { content: none; }
  > ._list-dir { line-height: 1.375rem; }

  ._list-arrow {
    left: 1rem;
    padding: .4375rem;
  }
}

//
// List pagination
//

._list-pagelink {
  color: var(--linkColor);
  cursor: pointer;

  &:hover {
    color: var(--linkColorHover);
    text-decoration: underline;
  }
}

//
// Search results
//

._list-result.active {
  padding-right: 1.75rem;

  > ._list-reveal { display: block; }
  > ._list-count { display: none; }
}

._list-reveal {
  display: none;
  position: absolute;
  top: 0;
  bottom: 0;
  right: 0;
  width: 2rem;
  cursor: pointer;

  &:before {
    content: '';
    position: absolute;
    bottom: 50%;
    left: .75rem;
    width: .75rem;
    height: 1px;
    background: var(--transparentSelectionText);
    box-shadow: 0 -3px var(--transparentSelectionText), // top line
                0 3px var(--transparentSelectionText);  // bottom line
  }
}

//
// List note
//

._list-note {
  padding: .5rem .75rem;
  line-height: 1.25rem;
  font-size: .8125rem;
  color: var(--textColorLight);

  & + & { padding-top: 0; }
}

._list-note-link { cursor: pointer; }

//
// List hover clone
//

._list-hover.clone {
  position: fixed;
  overflow: visible;
  z-index: var(--hoverZ);
  left: 0;
  min-width: var(--sidebarWidth);
  padding: .25rem .75rem;
  pointer-events: none;
  -webkit-font-smoothing: subpixel-antialiased;
  -webkit-transform: translate3d(0, 0, 0);
          transform: translate3d(0, 0, 0);
  @extend %border-box;

  @include mobile { min-width: var(--sidebarMediumWidth); }

  > ._list-text { display: inline; }

  &:not(._list-result) {
    padding-left: 2.75rem;

    &:before { content: none; }
  }

  ._list-reveal, ._list-enable { display: none; }
}

//
// List picker
//

._list-picker {
  ._list-item { cursor: pointer; }
  ._list-sub > ._list-item { padding-left: 2.375rem; }
}

._list-picker-head {
  display: flex;
  justify-content: space-between;
  position: -webkit-sticky;
  position: sticky;
  top: 0;
  z-index: 1;
  margin-right: 1px;
  padding: .5rem .75rem .25rem .75rem;
  line-height: 1.5rem;
  font-size: .75rem;
  font-weight: var(--bolderFontWeight);
  color: var(--textColorLight);
  text-transform: uppercase;
  background: linear-gradient(to bottom, var(--sidebarBackground), var(--sidebarBackground) 75%, var(--transparentSidebarBackground));
  cursor: default;
}

._list-checkbox {
  position: absolute;
  top: .5rem;
  right: .75rem;
}

._list-link {
  display: block;
  padding: .75rem 0;
  font-size: .8125rem;
  text-align: center;
  @extend %external-link;

  &:after { visibility: hidden; }
  &:hover:after { visibility: visible; }
}