Fixes scrolling + improves Performance through assigning found $ elements to variables

app/assets/javascripts/right_sidebar.js

@@ -7,6 +7,13 @@ import Cookies from 'js-cookie';
     function Sidebar(currentUser) {
       this.toggleTodo = this.toggleTodo.bind(this);
       this.sidebar = $('aside');
+
+      this.$sidebarInner = this.sidebar.find('.issuable-sidebar');
+      this.$navGitlab = $('.navbar-gitlab');
+      this.$layoutNav = $('.layout-nav');
+      this.$subScroll = $('.sub-nav-scroll');
+      this.$rightSidebar = $('.js-right-sidebar');
+
       this.removeListeners();
       this.addEventListeners();
     }
@@ -21,14 +28,15 @@ import Cookies from 'js-cookie';
     Sidebar.prototype.addEventListeners = function() {
       const $document = $(document);
-      const throttledSetSidebarHeight = _.throttle(this.setSidebarHeight, 10);
+      const throttledSetSidebarHeight = _.throttle(this.setSidebarHeight.bind(this), 20);
+      const debouncedSetSidebarHeight = _.debounce(this.setSidebarHeight.bind(this), 200);
       this.sidebar.on('click', '.sidebar-collapsed-icon', this, this.sidebarCollapseClicked);
       $('.dropdown').on('hidden.gl.dropdown', this, this.onSidebarDropdownHidden);
       $('.dropdown').on('loading.gl.dropdown', this.sidebarDropdownLoading);
       $('.dropdown').on('loaded.gl.dropdown', this.sidebarDropdownLoaded);
       $(window).on('resize', () => throttledSetSidebarHeight());
-      $document.on('scroll', () => throttledSetSidebarHeight());
+      $document.on('scroll', () => debouncedSetSidebarHeight());
       $document.on('click', '.js-sidebar-toggle', function(e, triggered) {
         var $allGutterToggleIcons, $this, $thisIcon;
         e.preventDefault();
@@ -207,13 +215,14 @@ import Cookies from 'js-cookie';
     };
     Sidebar.prototype.setSidebarHeight = function() {
-      const $navHeight = $('.navbar-gitlab').outerHeight() + $('.layout-nav').outerHeight() + $('.sub-nav-scroll').outerHeight();
-      const $rightSidebar = $('.js-right-sidebar');
+      const $navHeight = this.$navGitlab.outerHeight() + this.$layoutNav.outerHeight() + (this.$subScroll ? this.$subScroll.outerHeight() : 0);
       const diff = $navHeight - $(window).scrollTop();
       if (diff > 0) {
-        $rightSidebar.outerHeight($(window).height() - diff);
+        this.$rightSidebar.outerHeight($(window).height() - diff);
+        this.$sidebarInner.height('100%');
       } else {
-        $rightSidebar.outerHeight('100%');
+        this.$rightSidebar.outerHeight('100%');
+        this.$sidebarInner.height('');
       }
     };

app/assets/stylesheets/pages/issuable.scss

@@ -204,7 +204,7 @@
   .issuable-sidebar {
     width: calc(100% + 100px);
-    height: 100%;
+    height: calc(100% - #{$header-height});
     overflow-y: scroll;
     overflow-x: hidden;
     -webkit-overflow-scrolling: touch;