fix(material/slide-toggle): remove tabindex from host node (#23891)

The `tabindex="-1"` on the host node was causing VoicerOver to read out the slide toggle as a group and to read out the label twice. These changes remove the `tabindex` like we've done for `mat-radio` and `mat-checkbox` in the past.

(cherry picked from commit b533e61)

src/material-experimental/mdc-slide-toggle/slide-toggle.spec.ts

@@ -287,16 +287,6 @@ describe('MDC-based MatSlideToggle without forms', () => {
       expect(document.activeElement).toBe(buttonElement);
     }));
 
-    it('should focus on underlying element when the host is focused', fakeAsync(() => {
-      expect(document.activeElement).not.toBe(buttonElement);
-
-      slideToggleElement.focus();
-      fixture.detectChanges();
-      tick();
-
-      expect(document.activeElement).toBe(buttonElement);
-    }));
-
     it('should not manually move focus to underlying when focus comes from mouse or touch', fakeAsync(
       inject([FocusMonitor], (focusMonitor: FocusMonitor) => {
         expect(document.activeElement).not.toBe(buttonElement);
@@ -397,13 +387,13 @@ describe('MDC-based MatSlideToggle without forms', () => {
       expect(switchEl.classList).toContain('mdc-switch--checked');
     }));
 
-    it('should set the tabindex of the host element to -1', fakeAsync(() => {
+    it('should remove the tabindex from the host node', fakeAsync(() => {
       const fixture = TestBed.createComponent(SlideToggleWithTabindexAttr);
 
       fixture.detectChanges();
 
       const slideToggle = fixture.debugElement.query(By.directive(MatSlideToggle))!.nativeElement;
-      expect(slideToggle.getAttribute('tabindex')).toBe('-1');
+      expect(slideToggle.hasAttribute('tabindex')).toBe(false);
     }));
 
     it('should remove the tabindex from the host element when disabled', fakeAsync(() => {

src/material-experimental/mdc-slide-toggle/slide-toggle.ts

@@ -66,8 +66,8 @@ export class MatSlideToggleChange {
   host: {
     'class': 'mat-mdc-slide-toggle',
     '[id]': 'id',
-    // Needs to be `-1` so it can still receive programmatic focus.
-    '[attr.tabindex]': 'disabled ? null : -1',
+    // Needs to be removed since it causes some a11y issues (see #21266).
+    '[attr.tabindex]': 'null',
     '[attr.aria-label]': 'null',
     '[attr.aria-labelledby]': 'null',
     '[class.mat-primary]': 'color === "primary"',
@@ -221,12 +221,7 @@ export class MatSlideToggle implements ControlValueAccessor, AfterViewInit, OnDe
     foundation.setChecked(this.checked);
 
     this._focusMonitor.monitor(this._elementRef, true).subscribe(focusOrigin => {
-      // Only forward focus manually when it was received programmatically or through the
-      // keyboard. We should not do this for mouse/touch focus for two reasons:
-      // 1. It can prevent clicks from landing in Chrome (see #18269).
-      // 2. They're already handled by the wrapping `label` element.
       if (focusOrigin === 'keyboard' || focusOrigin === 'program') {
-        this._switchElement.nativeElement.focus();
         this._focused = true;
       } else if (!focusOrigin) {
         // When a focused element becomes disabled, the browser *immediately* fires a blur event.

src/material/slide-toggle/slide-toggle.spec.ts

@@ -318,16 +318,6 @@ describe('MatSlideToggle without forms', () => {
       expect(document.activeElement).toBe(inputElement);
     }));
 
-    it('should focus on underlying element when the host is focused', fakeAsync(() => {
-      expect(document.activeElement).not.toBe(inputElement);
-
-      slideToggleElement.focus();
-      fixture.detectChanges();
-      flush();
-
-      expect(document.activeElement).toBe(inputElement);
-    }));
-
     it('should not manually move focus to underlying when focus comes from mouse or touch', inject(
       [FocusMonitor],
       (focusMonitor: FocusMonitor) => {
@@ -410,13 +400,13 @@ describe('MatSlideToggle without forms', () => {
         .toBe(5);
     }));
 
-    it('should set the tabindex of the host element to -1', fakeAsync(() => {
+    it('should remove the tabindex from the host node', fakeAsync(() => {
       const fixture = TestBed.createComponent(SlideToggleWithTabindexAttr);
 
       fixture.detectChanges();
 
       const slideToggle = fixture.debugElement.query(By.directive(MatSlideToggle))!.nativeElement;
-      expect(slideToggle.getAttribute('tabindex')).toBe('-1');
+      expect(slideToggle.hasAttribute('tabindex')).toBe(false);
     }));
 
     it('should remove the tabindex from the host element when disabled', fakeAsync(() => {

src/material/slide-toggle/slide-toggle.ts

@@ -83,8 +83,8 @@ const _MatSlideToggleBase = mixinTabIndex(
   host: {
     'class': 'mat-slide-toggle',
     '[id]': 'id',
-    // Needs to be `-1` so it can still receive programmatic focus.
-    '[attr.tabindex]': 'disabled ? null : -1',
+    // Needs to be removed since it causes some a11y issues (see #21266).
+    '[attr.tabindex]': 'null',
     '[attr.aria-label]': 'null',
     '[attr.aria-labelledby]': 'null',
     '[class.mat-checked]': 'checked',
@@ -198,13 +198,7 @@ export class MatSlideToggle
 
   ngAfterContentInit() {
     this._focusMonitor.monitor(this._elementRef, true).subscribe(focusOrigin => {
-      // Only forward focus manually when it was received programmatically or through the
-      // keyboard. We should not do this for mouse/touch focus for two reasons:
-      // 1. It can prevent clicks from landing in Chrome (see #18269).
-      // 2. They're already handled by the wrapping `label` element.
-      if (focusOrigin === 'keyboard' || focusOrigin === 'program') {
-        this._inputElement.nativeElement.focus();
-      } else if (!focusOrigin) {
+      if (!focusOrigin) {
         // When a focused element becomes disabled, the browser *immediately* fires a blur event.
         // Angular does not expect events to be raised during change detection, so any state
         // change (such as a form control's 'ng-touched') will cause a changed-after-checked