Merge branch 'sticky-perf' into 'master'

Fix performance of sticky.js

Closes #39332

See merge request gitlab-org/gitlab-ce!14976

app/assets/javascripts/dispatcher.js

@@ -240,7 +240,8 @@ import Diff from './diff';
           break;
         case 'projects:compare:show':
           new Diff();
-          initChangesDropdown();
+          const paddingTop = 16;
+          initChangesDropdown(document.querySelector('.navbar-gitlab').offsetHeight - paddingTop);
           break;
         case 'projects:branches:new':
         case 'projects:branches:create':

app/assets/javascripts/init_changes_dropdown.js

 import stickyMonitor from './lib/utils/sticky';
 
-export default () => {
-  stickyMonitor(document.querySelector('.js-diff-files-changed'));
+export default (stickyTop) => {
+  stickyMonitor(document.querySelector('.js-diff-files-changed'), stickyTop);
 
   $('.js-diff-stats-dropdown').glDropdown({
     filterable: true,

app/assets/javascripts/lib/utils/sticky.js

@@ -28,14 +28,10 @@ export const isSticky = (el, scrollY, stickyTop, insertPlaceholder) => {
   }
 };
 
-export default (el, insertPlaceholder = true) => {
+export default (el, stickyTop, insertPlaceholder = true) => {
   if (!el) return;
 
-  const computedStyle = window.getComputedStyle(el);
-
-  if (!/sticky/.test(computedStyle.position)) return;
-
-  const stickyTop = parseInt(computedStyle.top, 10);
+  if (typeof CSS === 'undefined' || !(CSS.supports('(position: -webkit-sticky) or (position: sticky)'))) return;
 
   document.addEventListener('scroll', () => isSticky(el, window.scrollY, stickyTop, insertPlaceholder), {
     passive: true,

app/assets/javascripts/merge_request_tabs.js

@@ -67,6 +67,10 @@ import Diff from './diff';
   class MergeRequestTabs {
 
     constructor({ action, setUrl, stubLocation } = {}) {
+      const mergeRequestTabs = document.querySelector('.js-tabs-affix');
+      const navbar = document.querySelector('.navbar-gitlab');
+      const paddingTop = 16;
+
       this.diffsLoaded = false;
       this.pipelinesLoaded = false;
       this.commitsLoaded = false;
@@ -76,6 +80,11 @@ import Diff from './diff';
       this.setCurrentAction = this.setCurrentAction.bind(this);
       this.tabShown = this.tabShown.bind(this);
       this.showTab = this.showTab.bind(this);
+      this.stickyTop = navbar ? navbar.offsetHeight - paddingTop : 0;
+
+      if (mergeRequestTabs) {
+        this.stickyTop += mergeRequestTabs.offsetHeight;
+      }
 
       if (stubLocation) {
         location = stubLocation;
@@ -278,7 +287,7 @@ import Diff from './diff';
           const $container = $('#diffs');
           $container.html(data.html);
 
-          initChangesDropdown();
+          initChangesDropdown(this.stickyTop);
 
           if (typeof gl.diffNotesCompileComponents !== 'undefined') {
             gl.diffNotesCompileComponents();