fix(datepicker): inconsistent focus restoration timing in touchUi mode

`MatDatepicker` has focus restoration which happens (almost) synchronously, whereas `MatDialog` restores focus asynchronously when the animation is finished. This means that in `touchUi` mode focus ends up being restored both by the datepicker and the dialog within a 150ms window. The problem is that if the consumer of `MatDatepicker` decides to move focus inside the `closed` event, it'll be ovewritten by the dialog 150ms later. These changes disable the dialog's focus restoration because it's unnecessary.

Fixes #17560.

Author: crisbeto

src/material/datepicker/datepicker.spec.ts

@@ -976,6 +976,45 @@ describe('MatDatepicker', () => {
         expect(document.activeElement).toBe(toggle, 'Expected focus to be restored to toggle.');
       });
 
+      it('should not override focus if it was moved inside the closed event in touchUI mode',
+        fakeAsync(() => {
+          const focusTarget = document.createElement('button');
+          const datepicker = fixture.componentInstance.datepicker;
+          const subscription = datepicker.closedStream.subscribe(() => focusTarget.focus());
+          const input = fixture.nativeElement.querySelector('input');
+
+          focusTarget.setAttribute('tabindex', '0');
+          document.body.appendChild(focusTarget);
+
+          // Important: we're testing the touchUI behavior on particular.
+          fixture.componentInstance.touchUI = true;
+          fixture.detectChanges();
+
+          // Focus the input before opening so that the datepicker restores focus to it on close.
+          input.focus();
+
+          expect(document.activeElement).toBe(input, 'Expected input to be focused on init.');
+
+          datepicker.open();
+          fixture.detectChanges();
+          tick(500);
+          fixture.detectChanges();
+
+          expect(document.activeElement)
+              .not.toBe(input, 'Expected input not to be focused while dialog is open.');
+
+          datepicker.close();
+          fixture.detectChanges();
+          tick(500);
+          fixture.detectChanges();
+
+          expect(document.activeElement)
+              .toBe(focusTarget, 'Expected alternate focus target to be focused after closing.');
+
+          focusTarget.parentNode!.removeChild(focusTarget);
+          subscription.unsubscribe();
+        }));
+
       it('should re-render when the i18n labels change',
         inject([MatDatepickerIntl], (intl: MatDatepickerIntl) => {
           const toggle = fixture.debugElement.query(By.css('button'))!.nativeElement;

src/material/datepicker/datepicker.ts

@@ -405,6 +405,14 @@ export class MatDatepicker<D> implements OnDestroy, CanColor {
       direction: this._dir ? this._dir.value : 'ltr',
       viewContainerRef: this._viewContainerRef,
       panelClass: 'mat-datepicker-dialog',
+
+      // `MatDialog` has focus restoration built in, however we want to disable it since the
+      // datepicker also has focus restoration for dropdown mode. We want to do this, in order
+      // to ensure that the timing is consistent between dropdown and dialog modes since `MatDialog`
+      // restores focus when the animation is finished, but the datepicker does it immediately.
+      // Furthermore, this avoids any conflicts where the datepicker consumer might move focus
+      // inside the `closed` event which is dispatched immediately.
+      restoreFocus: false
     });
 
     this._dialogRef.afterClosed().subscribe(() => this.close());