fix(material/core): remove tabindex from mat-option

Remove the tabindex attribute added to MatOption components. MatOption
does not need tabindex because the parent component manages focus by
setting `aria-activedescendant` attribute. Previously, MatOption set
tabindex but was also a referenced by aria-activedescendant. This was
not the correct ARIA semantics. Align closer to ARIA spec by using only
aria-activedescendant rather than both.

Tabindex="-1" seems to be causing a problem in #26861 where VoiceOver
with Firefox moves DOM focus from the combobox to the option when
opening the listbox popup.

Unblocks #26861.

src/material/core/option/option.ts

@@ -204,11 +204,6 @@ export class _MatOptionBase<T = any> implements FocusableOption, AfterViewChecke
     }
   }
 
-  /** Returns the correct tabindex for the option depending on disabled state. */
-  _getTabIndex(): string {
-    return this.disabled ? '-1' : '0';
-  }
-
   /** Gets the host DOM element. */
   _getHostElement(): HTMLElement {
     return this._element.nativeElement;
@@ -251,7 +246,6 @@ export class _MatOptionBase<T = any> implements FocusableOption, AfterViewChecke
   exportAs: 'matOption',
   host: {
     'role': 'option',
-    '[attr.tabindex]': '_getTabIndex()',
     '[class.mdc-list-item--selected]': 'selected',
     '[class.mat-mdc-option-multiple]': 'multiple',
     '[class.mat-mdc-option-active]': 'active',

src/material/legacy-core/option/option.ts

@@ -59,4 +59,9 @@ export class MatLegacyOption<T = any> extends _MatOptionBase<T> {
   ) {
     super(element, changeDetectorRef, parent, group);
   }
+
+  /** Returns the correct tabindex for the option depending on disabled state. */
+  _getTabIndex(): string {
+    return this.disabled ? '-1' : '0';
+  }
 }

src/material/legacy-select/select.ts

@@ -489,7 +489,7 @@ export class MatLegacySelect extends _MatSelectBase<MatLegacySelectChange> imple
       selectedOptionOffset = 0;
     } else {
       selectedOptionOffset = Math.max(
-        this.options.toArray().indexOf(this._selectionModel.selected[0]),
+        this.options.toArray().indexOf(this._selectionModel.selected[0] as any),
         0,
       );
     }

src/material/select/select.spec.ts

@@ -824,17 +824,27 @@ describe('MDC-based MatSelect', () => {
               'mat-option',
             ) as NodeListOf<HTMLElement>;
 
-            options[3].focus();
+            select.focus();
+            multiFixture.detectChanges();
+            multiFixture.componentInstance.select._keyManager.setActiveItem(3);
+            multiFixture.detectChanges();
+
             expect(document.activeElement)
-              .withContext('Expected fourth option to be focused.')
-              .toBe(options[3]);
+              .withContext('Expected select to have DOM focus.')
+              .toBe(select);
+            expect(select.getAttribute('aria-activedescendant'))
+              .withContext('Expected fourth option to be activated.')
+              .toBe(options[3].id);
 
             multiFixture.componentInstance.control.setValue(['steak-0', 'sushi-7']);
             multiFixture.detectChanges();
 
             expect(document.activeElement)
-              .withContext('Expected fourth option to remain focused.')
-              .toBe(options[3]);
+              .withContext('Expected select to have DOM focus.')
+              .toBe(select);
+            expect(select.getAttribute('aria-activedescendant'))
+              .withContext('Expected fourth optino to remain activated.')
+              .toBe(options[3].id);
           }),
         );
 
@@ -1260,10 +1270,10 @@ describe('MDC-based MatSelect', () => {
             .toBe(true);
         }));
 
-        it('should set the tabindex of each option according to disabled state', fakeAsync(() => {
-          expect(options[0].getAttribute('tabindex')).toEqual('0');
-          expect(options[1].getAttribute('tabindex')).toEqual('0');
-          expect(options[2].getAttribute('tabindex')).toEqual('-1');
+        it('should omit the tabindex attribute on each option', fakeAsync(() => {
+          expect(options[0].hasAttribute('tabindex')).toBeFalse();
+          expect(options[1].hasAttribute('tabindex')).toBeFalse();
+          expect(options[2].hasAttribute('tabindex')).toBeFalse();
         }));
 
         it('should set aria-disabled for disabled options', fakeAsync(() => {

tools/public_api_guard/material/core.md

@@ -281,7 +281,6 @@ export class _MatOptionBase<T = any> implements FocusableOption, AfterViewChecke
     focus(_origin?: FocusOrigin, options?: FocusOptions): void;
     _getHostElement(): HTMLElement;
     getLabel(): string;
-    _getTabIndex(): string;
     // (undocumented)
     readonly group: _MatOptgroupBase;
     _handleKeydown(event: KeyboardEvent): void;

tools/public_api_guard/material/legacy-core.md

@@ -173,6 +173,7 @@ export { _MatLegacyOptgroupBase }
 // @public @deprecated
 export class MatLegacyOption<T = any> extends _MatLegacyOptionBase<T> {
     constructor(element: ElementRef<HTMLElement>, changeDetectorRef: ChangeDetectorRef, parent: MatLegacyOptionParentComponent, group: MatLegacyOptgroup);
+    _getTabIndex(): string;
     // (undocumented)
     static ɵcmp: i0.ɵɵComponentDeclaration<MatLegacyOption<any>, "mat-option", ["matOption"], {}, {}, never, ["*"], false, never>;
     // (undocumented)