fix(material/bottom-sheet): focus restoration not working inside shadow dom (#21975)

Related to #21796. The bottom sheet focus restoration works by grabbing `document.activeElement`
before the sheet is opened and restoring focus to the element on destroy. This won't work if the
element is inside the shadow DOM, because the browser will return the shadow root.
These changes add a workaround.

Author: crisbeto

src/material/bottom-sheet/BUILD.bazel

@@ -58,6 +58,7 @@ ng_test_library(
         "//src/cdk/bidi",
         "//src/cdk/keycodes",
         "//src/cdk/overlay",
+        "//src/cdk/platform",
         "//src/cdk/scrolling",
         "//src/cdk/testing/private",
         "@npm//@angular/common",

src/material/bottom-sheet/bottom-sheet-container.ts

@@ -207,7 +207,7 @@ export class MatBottomSheetContainer extends BasePortalOutlet implements OnDestr
     if (this.bottomSheetConfig.autoFocus) {
       this._focusTrap.focusInitialElementWhenReady();
     } else {
-      const activeElement = this._document.activeElement;
+      const activeElement = this._getActiveElement();
 
       // Otherwise ensure that focus is on the container. It's possible that a different
       // component tried to move focus while the open animation was running. See:
@@ -226,7 +226,7 @@ export class MatBottomSheetContainer extends BasePortalOutlet implements OnDestr
 
     // We need the extra check, because IE can set the `activeElement` to null in some cases.
     if (this.bottomSheetConfig.restoreFocus && toFocus && typeof toFocus.focus === 'function') {
-      const activeElement = this._document.activeElement;
+      const activeElement = this._getActiveElement();
       const element = this._elementRef.nativeElement;
 
       // Make sure that focus is still inside the bottom sheet or is on the body (usually because a
@@ -246,11 +246,19 @@ export class MatBottomSheetContainer extends BasePortalOutlet implements OnDestr
 
   /** Saves a reference to the element that was focused before the bottom sheet was opened. */
   private _savePreviouslyFocusedElement() {
-    this._elementFocusedBeforeOpened = this._document.activeElement as HTMLElement;
+    this._elementFocusedBeforeOpened = this._getActiveElement();
 
     // The `focus` method isn't available during server-side rendering.
     if (this._elementRef.nativeElement.focus) {
       Promise.resolve().then(() => this._elementRef.nativeElement.focus());
     }
   }
+
+  /** Gets the currently-focused element on the page. */
+  private _getActiveElement(): HTMLElement | null {
+    // If the `activeElement` is inside a shadow root, `document.activeElement` will
+    // point to the shadow root so we have to descend into it ourselves.
+    const activeElement = this._document.activeElement;
+    return activeElement?.shadowRoot?.activeElement as HTMLElement || activeElement;
+  }
 }

src/material/bottom-sheet/bottom-sheet.spec.ts

@@ -1,6 +1,7 @@
 import {Directionality} from '@angular/cdk/bidi';
 import {A, ESCAPE} from '@angular/cdk/keycodes';
 import {OverlayContainer, ScrollStrategy} from '@angular/cdk/overlay';
+import {_supportsShadowDom} from '@angular/cdk/platform';
 import {ViewportRuler} from '@angular/cdk/scrolling';
 import {
   dispatchKeyboardEvent,
@@ -18,6 +19,7 @@ import {
   TemplateRef,
   ViewChild,
   ViewContainerRef,
+  ViewEncapsulation,
 } from '@angular/core';
 import {
   ComponentFixture,
@@ -28,6 +30,7 @@ import {
   TestBed,
   tick,
 } from '@angular/core/testing';
+import {By} from '@angular/platform-browser';
 import {NoopAnimationsModule} from '@angular/platform-browser/animations';
 
 import {MAT_BOTTOM_SHEET_DEFAULT_OPTIONS, MatBottomSheet} from './bottom-sheet';
@@ -741,6 +744,33 @@ describe('MatBottomSheet', () => {
       body.removeChild(otherButton);
     }));
 
+    it('should re-focus trigger element inside the shadow DOM when the bottom sheet is dismissed',
+      fakeAsync(() => {
+        if (!_supportsShadowDom()) {
+          return;
+        }
+
+        viewContainerFixture.destroy();
+        const fixture = TestBed.createComponent(ShadowDomComponent);
+        fixture.detectChanges();
+        const button = fixture.debugElement.query(By.css('button'))!.nativeElement;
+
+        button.focus();
+
+        const ref = bottomSheet.open(PizzaMsg);
+        flushMicrotasks();
+        fixture.detectChanges();
+        flushMicrotasks();
+
+        const spy = spyOn(button, 'focus').and.callThrough();
+        ref.dismiss();
+        flushMicrotasks();
+        fixture.detectChanges();
+        tick(500);
+
+        expect(spy).toHaveBeenCalled();
+      }));
+
   });
 
 });
@@ -954,6 +984,12 @@ class BottomSheetWithInjectedData {
   constructor(@Inject(MAT_BOTTOM_SHEET_DATA) public data: any) { }
 }
 
+@Component({
+  template: `<button>I'm a button</button>`,
+  encapsulation: ViewEncapsulation.ShadowDom
+})
+class ShadowDomComponent {}
+
 // Create a real (non-test) NgModule as a workaround for
 // https://github.com/angular/angular/issues/10760
 const TEST_DIRECTIVES = [
@@ -963,6 +999,7 @@ const TEST_DIRECTIVES = [
   TacoMsg,
   DirectiveWithViewContainer,
   BottomSheetWithInjectedData,
+  ShadowDomComponent,
 ];
 
 @NgModule({