fix(common): prevent focus from scrollToAnchor

Focus the target element using `focus({preventScroll: true})` after scrolling, so the browser doesn’t adjust the scroll position when applying focus.

Fixes #65938

(cherry picked from commit 97cac1cf4d)

packages/common/src/viewport_scroller.ts

@@ -55,6 +55,7 @@ export abstract class ViewportScroller {
   /**
    * Scrolls to an anchor element.
    * @param anchor The ID of the anchor element.
+   * @param options Scroll options
    */
   abstract scrollToAnchor(anchor: string, options?: ScrollOptions): void;
 
@@ -125,11 +126,12 @@ export class BrowserViewportScroller implements ViewportScroller {
       this.scrollToElement(elSelected, options);
       // After scrolling to the element, the spec dictates that we follow the focus steps for the
       // target. Rather than following the robust steps, simply attempt focus.
-      //
+      // Use `preventScroll: true` to avoid extra scroll that breaks smooth scrolling.
       // @see https://html.spec.whatwg.org/#get-the-focusable-area
       // @see https://developer.mozilla.org/en-US/docs/Web/API/HTMLOrForeignElement/focus
       // @see https://html.spec.whatwg.org/#focusable-area
-      elSelected.focus();
+      // @see https://www.yanandcoffee.com/2020/05/08/accessible-smooth-scrolling-and-focus-management-solutions/
+      elSelected.focus({preventScroll: true});
     }
   }
 
@@ -235,7 +237,7 @@ export class NullViewportScroller implements ViewportScroller {
   /**
    * Empty implementation
    */
-  scrollToAnchor(anchor: string): void {}
+  scrollToAnchor(anchor: string, options?: ScrollOptions): void {}
 
   /**
    * Empty implementation

packages/common/test/viewport_scroller_spec.ts

@@ -7,7 +7,7 @@
  */
 
 import {BrowserViewportScroller, ViewportScroller} from '../src/viewport_scroller';
-import {isNode} from '@angular/private/testing';
+import {isNode, waitFor} from '@angular/private/testing';
 
 describe('BrowserViewportScroller', () => {
   describe('setHistoryScrollRestoration', () => {
@@ -112,6 +112,30 @@ describe('BrowserViewportScroller', () => {
       cleanup();
     });
 
+    it('should honor the scroll offset when smooth scrolling', async () => {
+      // Ensure the scroll behavior is smooth for this test, as the bug only occurred with smooth scrolling.
+      document.documentElement.style.scrollBehavior = 'smooth';
+
+      const {anchorNode, cleanup} = createTallElement();
+      anchorNode.id = anchor;
+
+      // Padding ensures the page is tall enough that the offset-adjusted target
+      // is reachable and not clamped to the maximum scroll position.
+      document.body.style.paddingBottom = '5000px';
+      // Header offset
+      scroller.setOffset([0, 80]);
+
+      scroller.scrollToAnchor(anchor);
+
+      await waitFor(() => throwUnless(anchorNode.getBoundingClientRect().top).toBe(80), {
+        timeout: 1_000,
+      });
+
+      document.documentElement.style.scrollBehavior = '';
+      document.body.style.paddingBottom = '';
+      cleanup();
+    });
+
     function createTallElement() {
       const tallItem = document.createElement('div');
       tallItem.style.height = '3000px';