devdocs/assets/stylesheets/components/_header.scss

//
// Header
//

._header {
  position: absolute;
  z-index: $headerZ;
  top: 0;
  left: 0;
  right: 0;
  height: $headerHeight;
  line-height: $headerHeight;
  background: $headerBackground;
  border-bottom: 1px solid $headerBorder;
  @extend %user-select-none;

  a:focus { outline: 0; }
}

//
// Navigation menu
//

._nav {
  float: right;
  margin-right: .5rem;
  font-size: .875rem;
  color: $textColor;
}

._nav-link,
._nav-link:hover {
  position: relative;
  float: left;
  padding: 0 1.25rem;
  color: inherit;
  text-decoration: none;
}

._nav-link {
  &:before, &:after {
    position: absolute;
    left: 50%;
    bottom: 0;
    width: 0;
    height: 0;
    margin-left: -.375rem;
    border: .375rem solid transparent;
    border-bottom-color: darken($headerBorder, 2%);
  }

  &:after {
    bottom: -1px;
    border-bottom-color: $contentBackground;
  }
}

._nav-current {
  &:before, &:after { content: ''; }
}

//
// Logo
//

._logo {
  position: relative;
  float: left;
  height: $headerHeight;
  margin: 0;
  line-height: inherit;
  font-size: inherit;

  > ._nav-link {
    float: none;
    padding: 0 1rem;
  }
}

//
// Search form
//

._search {
  position: relative;
  float: left;
  width: $sidebarWidth;
  height: 100%;
  padding: .5rem 0 .5rem .5rem;
  @extend %border-box;

  @media #{$mediumScreen} { width: $sidebarMediumWidth; }

  &:before {
    position: absolute;
    top: 1rem;
    left: 1rem;
    opacity: .4;
    @extend %icon, %icon-search;
  }
}

._search-input {
  display: block;
  width: 100%;
  height: 100%;
  padding: 0 .75rem 1px 1.75rem;
  font-size: .875rem;
  background: $contentBackground;
  border: 1px solid;
  border-color: darken($headerBorder, 2%);
  border-radius: 3px;

  &:focus {
    outline: 0;
    border-color: $inputFocusBorder;
    box-shadow: 0 0 1px $inputFocusBorder;
  }
}

._search-clear {
  display: none;
  position: absolute;
  top: .5em;
  right: 0;
  padding: .5em;
  cursor: pointer;
  opacity: .3;

  &:hover { opacity: .5; }
  &:before { @extend %icon, %icon-clear; }
  ._search-active > & { display: block; }
}

._search-tag {
  display: none;
  position: absolute;
  top: .875rem;
  left: .875rem;
  padding: 0 .5rem;
  line-height: 1.25rem;
  font-size: .875rem;
  color: $textColorLight;
  background: darken($headerBackground, 5%);
  border-radius: 2px;
}