The basis for refactoring of the styles

We create a duplication of the existing stylesheet and prefix the
styles in it with the CSS class assigned on body in case the FF is
enabled

app/assets/stylesheets/framework.scss

@@ -39,7 +39,7 @@
 @import 'framework/selects';
 @import 'framework/sidebar';
 @import 'framework/contextual_sidebar_header';
-@import 'framework/contextual_sidebar';
+@import 'framework/contextual_sidebar_refactoring/contextual_sidebar';
 @import 'framework/tables';
 @import 'framework/notes';
 @import 'framework/tabs';

app/assets/stylesheets/framework/contextual_sidebar_refactoring/contextual_sidebar.scss

+body:not(.sidebar-refactoring) {
+  @import 'contextual_sidebar_base';
+}
+
+body.sidebar-refactoring {
+  @import 'contextual_sidebar_variant';
+}

app/assets/stylesheets/framework/contextual_sidebar_refactoring/contextual_sidebar_variant.scss

+.page-with-contextual-sidebar {
+  transition: padding-left $sidebar-transition-duration;
+
+  @include media-breakpoint-up(md) {
+    padding-left: $contextual-sidebar-collapsed-width;
+  }
+
+  @include media-breakpoint-up(xl) {
+    padding-left: $contextual-sidebar-width;
+  }
+
+  .issues-bulk-update.right-sidebar.right-sidebar-expanded .issuable-sidebar-header {
+    padding: 10px 0 15px;
+  }
+}
+
+.page-with-icon-sidebar {
+  @include media-breakpoint-up(md) {
+    padding-left: $contextual-sidebar-collapsed-width;
+  }
+}
+
+.settings-avatar {
+  background-color: $white;
+
+  svg {
+    fill: $gl-text-color-secondary;
+    margin: auto;
+  }
+}
+
+@mixin collapse-contextual-sidebar-content {
+
+  @include context-header-collapsed;
+
+  .sidebar-top-level-items > li {
+    .sidebar-sub-level-items {
+      &:not(.flyout-list) {
+        display: none;
+      }
+    }
+  }
+
+  .nav-icon-container {
+    margin-right: 0;
+  }
+
+  .toggle-sidebar-button {
+    padding: 16px;
+    width: $contextual-sidebar-collapsed-width - 1px;
+
+    .collapse-text,
+    .icon-chevron-double-lg-left {
+      display: none;
+    }
+
+    .icon-chevron-double-lg-right {
+      display: block;
+      margin: 0;
+    }
+  }
+}
+
+.nav-sidebar {
+  transition: width $sidebar-transition-duration, left $sidebar-transition-duration;
+  position: fixed;
+  z-index: 600;
+  width: $contextual-sidebar-width;
+  top: $header-height;
+  bottom: 0;
+  left: 0;
+  background-color: $gray-light;
+  box-shadow: inset -1px 0 0 $border-color;
+  transform: translate3d(0, 0, 0);
+
+  &:not(.sidebar-collapsed-desktop) {
+    @media (min-width: map-get($grid-breakpoints, sm)) and (max-width: map-get($grid-breakpoints, sm)) {
+      box-shadow: inset -1px 0 0 $border-color, 2px 1px 3px $dropdown-shadow-color;
+    }
+  }
+
+  @mixin collapse-contextual-sidebar {
+    width: $contextual-sidebar-collapsed-width;
+
+    .nav-sidebar-inner-scroll {
+      overflow-x: hidden;
+    }
+
+    .badge.badge-pill:not(.fly-out-badge),
+    .nav-item-name {
+      @include gl-sr-only;
+    }
+
+    .sidebar-top-level-items > li > a {
+      min-height: 45px;
+    }
+
+    .fly-out-top-item {
+      display: block;
+    }
+
+    .avatar-container {
+      margin: 0 auto;
+    }
+  }
+
+  &.sidebar-collapsed-desktop {
+    @include collapse-contextual-sidebar;
+  }
+
+  &.sidebar-expanded-mobile {
+    left: 0;
+  }
+
+  a {
+    text-decoration: none;
+  }
+
+  ul {
+    padding-left: 0;
+    list-style: none;
+  }
+
+  li {
+    white-space: nowrap;
+
+    a {
+      transition: padding $sidebar-transition-duration;
+      display: flex;
+      align-items: center;
+      padding: 12px $gl-padding;
+      color: $gl-text-color-secondary;
+    }
+
+    .nav-item-name {
+      flex: 1;
+    }
+
+    &.active {
+      > a {
+        font-weight: $gl-font-weight-bold;
+      }
+    }
+  }
+
+  @include media-breakpoint-down(sm) {
+    left: (-$contextual-sidebar-width);
+  }
+
+  .nav-icon-container {
+    display: flex;
+    margin-right: 8px;
+  }
+
+  .fly-out-top-item {
+    display: none;
+  }
+
+  svg {
+    height: 16px;
+    width: 16px;
+  }
+
+  @media (min-width: map-get($grid-breakpoints, md)) and (max-width: map-get($grid-breakpoints, xl) - 1px) {
+    &:not(.sidebar-expanded-mobile) {
+      @include collapse-contextual-sidebar;
+      @include collapse-contextual-sidebar-content;
+    }
+  }
+}
+
+.nav-sidebar-inner-scroll {
+  height: 100%;
+  width: 100%;
+  overflow: auto;
+}
+
+.with-performance-bar .nav-sidebar {
+  top: $header-height + $performance-bar-height;
+}
+
+.sidebar-sub-level-items {
+  display: none;
+  padding-bottom: 8px;
+
+  > li {
+    a {
+      padding: 8px 16px 8px 40px;
+
+      &:hover,
+      &:focus {
+        background: $link-active-background;
+        color: $gl-text-color;
+      }
+    }
+
+    &.active {
+      a {
+        &,
+        &:hover,
+        &:focus {
+          background: $link-active-background;
+        }
+      }
+    }
+  }
+}
+
+.sidebar-top-level-items {
+  margin-bottom: 60px;
+
+  > li {
+    > a {
+      @include media-breakpoint-up(sm) {
+        margin-right: 1px;
+      }
+
+      &:hover {
+        color: $gl-text-color;
+      }
+    }
+
+    &.is-showing-fly-out {
+      > a {
+        margin-right: 1px;
+      }
+
+      .sidebar-sub-level-items {
+        @include media-breakpoint-up(sm) {
+          position: fixed;
+          top: 0;
+          left: 0;
+          min-width: 150px;
+          margin-top: -1px;
+          padding: 4px 1px;
+          background-color: $white;
+          box-shadow: 2px 1px 3px $dropdown-shadow-color;
+          border: 1px solid $gray-darker;
+          border-left: 0;
+          border-radius: 0 3px 3px 0;
+
+          &::before {
+            content: '';
+            position: absolute;
+            top: -30px;
+            bottom: -30px;
+            left: -10px;
+            right: -30px;
+            z-index: -1;
+          }
+
+          &.is-above {
+            margin-top: 1px;
+          }
+
+          .divider {
+            height: 1px;
+            margin: 4px -1px;
+            padding: 0;
+            background-color: $dropdown-divider-bg;
+          }
+
+          > .active {
+            box-shadow: none;
+
+            > a {
+              background-color: transparent;
+            }
+          }
+
+          a {
+            padding: 8px 16px;
+            color: $gl-text-color;
+
+            &:hover,
+            &:focus {
+              background-color: $gray-darker;
+            }
+          }
+        }
+      }
+    }
+
+    .badge.badge-pill {
+      background-color: $inactive-badge-background;
+      color: $gl-text-color-secondary;
+    }
+
+    &.active {
+      background: $link-active-background;
+
+      > a {
+        margin-left: 4px;
+        // Subtract width of left border on active element
+        padding-left: $gl-padding-12;
+      }
+
+      .badge.badge-pill {
+        font-weight: $gl-font-weight-bold;
+      }
+
+      .sidebar-sub-level-items:not(.is-fly-out-only) {
+        display: block;
+      }
+    }
+
+    &.active > a:hover,
+    &.is-over > a {
+      background-color: $link-hover-background;
+    }
+  }
+}
+
+// Collapsed nav
+
+.toggle-sidebar-button,
+.close-nav-button {
+  @include side-panel-toggle;
+}
+
+.toggle-sidebar-button,
+.close-nav-button {
+  position: fixed;
+  bottom: 0;
+  width: $contextual-sidebar-width - 1px;
+  border-top: 1px solid $border-color;
+
+  svg {
+    margin-right: 8px;
+  }
+
+  .icon-chevron-double-lg-right {
+    display: none;
+  }
+}
+
+.collapse-text {
+  white-space: nowrap;
+  overflow: hidden;
+}
+
+.sidebar-collapsed-desktop {
+  @include collapse-contextual-sidebar-content;
+}
+
+.fly-out-top-item {
+  > a {
+    display: flex;
+  }
+
+  .fly-out-badge {
+    margin-left: 8px;
+  }
+}
+
+.fly-out-top-item-name {
+  flex: 1;
+}
+
+// Mobile nav
+
+.close-nav-button {
+  display: none;
+}
+
+@include media-breakpoint-down(sm) {
+  .close-nav-button {
+    display: flex;
+  }
+
+  .toggle-sidebar-button {
+    display: none;
+  }
+
+  .mobile-overlay {
+    display: none;
+
+    &.mobile-nav-open {
+      display: block;
+      position: fixed;
+      background-color: $black-transparent;
+      height: 100%;
+      width: 100%;
+      z-index: $zindex-dropdown-menu;
+    }
+  }
+}