fix(material/datepicker): focus restoration not working inside shadow dom (#21796)

Our focus restoration works by checking `document.activeElement` before the
panel is opened and restoring to that element on close. The problem is that
`activeElement` will return the shadow root, if the focused element is inside one.

These changes add some extra logic to account for it.

Fixes #21785.

Author: crisbeto

src/material/datepicker/BUILD.bazel

@@ -97,6 +97,7 @@ ng_test_library(
         "//src/cdk/bidi",
         "//src/cdk/keycodes",
         "//src/cdk/overlay",
+        "//src/cdk/platform",
         "//src/cdk/scrolling",
         "//src/cdk/testing/private",
         "//src/material/core",

src/material/datepicker/datepicker-base.ts

@@ -550,10 +550,12 @@ export abstract class MatDatepickerBase<C extends MatDatepickerControl<D>, S,
     if (!this.datepickerInput && (typeof ngDevMode === 'undefined' || ngDevMode)) {
       throw Error('Attempted to open an MatDatepicker with no associated input.');
     }
-    if (this._document) {
-      this._focusedElementBeforeOpen = this._document.activeElement;
-    }
 
+    // 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: HTMLElement|null = this._document?.activeElement;
+    this._focusedElementBeforeOpen =
+      activeElement?.shadowRoot?.activeElement as HTMLElement || activeElement;
     this.touchUi ? this._openAsDialog() : this._openAsPopup();
     this._opened = true;
     this.openedStream.emit();

src/material/datepicker/datepicker.spec.ts

@@ -9,7 +9,7 @@ import {
   dispatchKeyboardEvent,
   dispatchMouseEvent,
 } from '@angular/cdk/testing/private';
-import {Component, Type, ViewChild, Provider, Directive} from '@angular/core';
+import {Component, Type, ViewChild, Provider, Directive, ViewEncapsulation} from '@angular/core';
 import {ComponentFixture, fakeAsync, flush, inject, TestBed, tick} from '@angular/core/testing';
 import {
   FormControl,
@@ -27,6 +27,7 @@ import {
 import {MatFormField, MatFormFieldModule} from '@angular/material/form-field';
 import {DEC, JAN, JUL, JUN, SEP} from '@angular/material/testing';
 import {By} from '@angular/platform-browser';
+import {_supportsShadowDom} from '@angular/cdk/platform';
 import {BrowserDynamicTestingModule} from '@angular/platform-browser-dynamic/testing';
 import {NoopAnimationsModule} from '@angular/platform-browser/animations';
 import {MAT_DIALOG_DEFAULT_OPTIONS, MatDialogConfig} from '@angular/material/dialog';
@@ -1139,6 +1140,33 @@ describe('MatDatepicker', () => {
         expect(document.activeElement).toBe(toggle, 'Expected focus to be restored to toggle.');
       });
 
+      it('should restore focus when placed inside a shadow root', () => {
+        if (!_supportsShadowDom()) {
+          return;
+        }
+
+        fixture.destroy();
+        TestBed.resetTestingModule();
+        fixture = createComponent(DatepickerWithToggleInShadowDom, [MatNativeDateModule]);
+        fixture.detectChanges();
+        testComponent = fixture.componentInstance;
+
+        const toggle = fixture.debugElement.query(By.css('button'))!.nativeElement;
+        fixture.componentInstance.touchUI = false;
+        fixture.detectChanges();
+
+        toggle.focus();
+        spyOn(toggle, 'focus').and.callThrough();
+        fixture.componentInstance.datepicker.open();
+        fixture.detectChanges();
+        fixture.componentInstance.datepicker.close();
+        fixture.detectChanges();
+
+        // We have to assert by looking at the `focus` method, because
+        // `document.activeElement` will return the shadow root.
+        expect(toggle.focus).toHaveBeenCalled();
+      });
+
       it('should allow for focus restoration to be disabled', () => {
         let toggle = fixture.debugElement.query(By.css('button'))!.nativeElement;
 
@@ -2352,6 +2380,17 @@ class DatepickerWithToggle {
 }
 
 
+@Component({
+  encapsulation: ViewEncapsulation.ShadowDom,
+  template: `
+    <input [matDatepicker]="d">
+    <mat-datepicker-toggle [for]="d" [aria-label]="ariaLabel"></mat-datepicker-toggle>
+    <mat-datepicker #d [touchUi]="touchUI" [restoreFocus]="restoreFocus"></mat-datepicker>
+  `,
+})
+class DatepickerWithToggleInShadowDom extends DatepickerWithToggle {}
+
+
 @Component({
   template: `
     <input [matDatepicker]="d">