Fix scroll flicker

See https://gitlab.com/gitlab-org/gitlab-ce/issues/34407

 - Revert https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12399
 - Update https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12299
   throttle/debounce to happen immediately and cleanup

app/assets/javascripts/right_sidebar.js

@@ -10,6 +10,8 @@ import Cookies from 'js-cookie';
       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();
@@ -27,14 +29,14 @@ import Cookies from 'js-cookie';
     Sidebar.prototype.addEventListeners = function() {
       const $document = $(document);
       const throttledSetSidebarHeight = _.throttle(this.setSidebarHeight.bind(this), 20);
-      const debouncedSetSidebarHeight = _.debounce(this.setSidebarHeight.bind(this), 200);
+      const slowerThrottledSetSidebarHeight = _.throttle(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', () => debouncedSetSidebarHeight());
+      $document.on('scroll', () => slowerThrottledSetSidebarHeight());
       $document.on('click', '.js-sidebar-toggle', function(e, triggered) {
         var $allGutterToggleIcons, $this, $thisIcon;
         e.preventDefault();
@@ -213,7 +215,7 @@ import Cookies from 'js-cookie';
     };
     Sidebar.prototype.setSidebarHeight = function() {
-      const $navHeight = this.$navGitlab.outerHeight();
+      const $navHeight = this.$navGitlab.outerHeight() + this.$layoutNav.outerHeight() + (this.$subScroll ? this.$subScroll.outerHeight() : 0);
       const diff = $navHeight - $(window).scrollTop();
       if (diff > 0) {
         this.$rightSidebar.outerHeight($(window).height() - diff);