Make a responsive fixed-with subnav.

responsive-subnav.scss

//Requires bourbon for media query

$page-width: 1020px;
$subnav-width: 260px;
$gutter: 65px;

.wrapper {
    &.has-subnav {
      max-width: $page-width;
      > aside {
        width: $subnav-width;
        float: left;
        position: absolute;
        height: 100%;
        @include max-screen(767px) {
          display: none;
        }
      }
      > section {
        padding-left: $subnav-width + $gutter;
        @include max-screen(767px) {
          padding-left: 0;
        }
      }
    }
  }