Fixes the diff changes buttons from toggling when scrolling

Closes #36698

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

 export const isSticky = (el, scrollY, stickyTop) => {
-  const top = el.offsetTop - scrollY;
+  const top = Math.floor(el.offsetTop - scrollY);
 
   if (top <= stickyTop) {
     el.classList.add('is-stuck');

app/assets/javascripts/merge_request_tabs.js

@@ -253,6 +253,7 @@ import bp from './breakpoints';
 
     loadDiff(source) {
       if (this.diffsLoaded) {
+        document.dispatchEvent(new CustomEvent('scroll'));
         return;
       }
 

changelogs/unreleased/changes-bar-sticky-fix.yml

+---
+title: Fixed diff changes bar buttons from showing/hiding whilst scrolling
+merge_request:
+author:
+type: fixed

spec/javascripts/merge_request_tabs_spec.js

@@ -295,6 +295,17 @@ import 'vendor/jquery.scrollTo';
         this.class.loadDiff('/foo/bar/merge_requests/1/diffs');
       });
 
+      it('triggers scroll event when diff already loaded', function () {
+        spyOn(document, 'dispatchEvent');
+
+        this.class.diffsLoaded = true;
+        this.class.loadDiff('/foo/bar/merge_requests/1/diffs');
+
+        expect(
+          document.dispatchEvent,
+        ).toHaveBeenCalledWith(new CustomEvent('scroll'));
+      });
+
       describe('with inline diff', () => {
         let noteId;
         let noteLineNumId;