Clean up fixed navbar

app/assets/javascripts/layout_nav.js

@@ -48,8 +48,11 @@ import _ from 'underscore';
   });
 
   function applyScrollNavClass() {
-    $('.navbar-border').css('opacity', $(window).scrollTop() / 40);
+    const scrollOpacityHeight = 40;
+    $('.navbar-border').css('opacity', Math.min($(window).scrollTop() / scrollOpacityHeight, 1));
   }
 
-  $(window).scroll(_.throttle(applyScrollNavClass, 100));
+  $(() => {
+    $(window).on('scroll', _.throttle(applyScrollNavClass, 100));
+  });
 }).call(window);

app/assets/stylesheets/framework/header.scss

@@ -116,10 +116,10 @@ header {
 
   .navbar-border {
     height: 1px;
-    position: fixed;
-    left: 0;
+    position: absolute;
     right: 0;
-    top: 50px;
+    left: 0;
+    bottom: 0;
     background-color: $border-color;
     opacity: 0;
   }

app/assets/stylesheets/pages/merge_requests.scss

@@ -689,7 +689,7 @@
 }
 
 .merge-request-tabs-holder {
-  top: 0;
+  top: $header-height;
   z-index: 10;
   background-color: $white-light;