Fix performance of sticky.js

Closes #39332

app/assets/javascripts/init_changes_dropdown.js

 import stickyMonitor from './lib/utils/sticky';
 
 export default () => {
-  stickyMonitor(document.querySelector('.js-diff-files-changed'));
+  stickyMonitor(document.querySelector('.js-diff-files-changed'), 76);
 
   $('.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,