Remove fixed positioning from navbar

app/assets/stylesheets/framework/nav.scss

@@ -283,10 +283,7 @@
 }
 
 .layout-nav {
-  position: fixed;
-  top: $header-height;
   width: 100%;
-  z-index: 11;
   background: $gray-light;
   border-bottom: 1px solid $border-color;
   transition: padding $sidebar-transition-duration;
@@ -419,7 +416,6 @@
 }
 
 .page-with-layout-nav {
-  margin-top: $header-height + 2;
 
   .right-sidebar {
     top: ($header-height * 2) + 2;

app/assets/stylesheets/framework/sidebar.scss

 .page-with-sidebar {
-  padding: $header-height 0 25px;
+  padding-bottom: 25px;
   transition: padding $sidebar-transition-duration;
 
   &.page-sidebar-pinned {

app/views/layouts/header/_default.html.haml

-%header.navbar.navbar-fixed-top.navbar-gitlab{ class: nav_header_class }
+%header.navbar.navbar-gitlab{ class: nav_header_class }
   %a.sr-only.gl-accessibility{ href: "#content-body", tabindex: "1" } Skip to content
   .container-fluid
     .header-content

spec/javascripts/fixtures/header.html.haml

-%header.navbar.navbar-fixed-top.navbar-gitlab.nav_header_class
+%header.navbar.navbar-gitlab.nav_header_class
   .container-fluid
     .header-content
       %button.side-nav-toggle