fix(option): Remove aria-selected='false' in single-selection mode (#15617)

Add an Input to optionally remove the aria-selected attribute from unselected options.
The default behavior is unchanged. Motivation: the screen reader NVDA announces 'not selected'
on any element that has aria-selected='false', which is disruptive when a user is navigating
through a long list of options. The W3 aria best practices example only uses aria-selected='true',
false is implicit:
https://www.w3.org/TR/wai-aria-practices/examples/listbox/listbox-collapsible.html

src/lib/core/option/option.ts

@@ -72,7 +72,7 @@ export const MAT_OPTION_PARENT_COMPONENT =
     '[class.mat-option-multiple]': 'multiple',
     '[class.mat-active]': 'active',
     '[id]': 'id',
-    '[attr.aria-selected]': 'selected.toString()',
+    '[attr.aria-selected]': '_getAriaSelected()',
     '[attr.aria-disabled]': 'disabled.toString()',
     '[class.mat-option-disabled]': 'disabled',
     '(click)': '_selectViaInteraction()',
@@ -220,6 +220,16 @@ export class MatOption implements AfterViewChecked, OnDestroy {
     }
   }
 
+  /**
+   * Gets the `aria-selected` value for the option. We explicitly omit the `aria-selected`
+   * attribute from single-selection, unselected options. Including the `aria-selected="false"`
+   * attributes adds a significant amount of noise to screen-reader users without providing useful
+   * information.
+   */
+  _getAriaSelected(): boolean|null {
+    return this.selected || (this.multiple ? false : null);
+  }
+
   /** Returns the correct tabindex for the option depending on disabled state. */
   _getTabIndex(): string {
     return this.disabled ? '-1' : '0';

src/lib/select/select.spec.ts

@@ -863,7 +863,7 @@ describe('MatSelect', () => {
       describe('for options', () => {
         let fixture: ComponentFixture<BasicSelect>;
         let trigger: HTMLElement;
-        let options: NodeListOf<HTMLElement>;
+        let options: Array<HTMLElement>;
 
         beforeEach(fakeAsync(() => {
           fixture = TestBed.createComponent(BasicSelect);
@@ -872,8 +872,7 @@ describe('MatSelect', () => {
           trigger.click();
           fixture.detectChanges();
 
-          options =
-              overlayContainerElement.querySelectorAll('mat-option') as NodeListOf<HTMLElement>;
+          options = Array.from(overlayContainerElement.querySelectorAll('mat-option'));
         }));
 
         it('should set the role of mat-option to option', fakeAsync(() => {
@@ -882,10 +881,9 @@ describe('MatSelect', () => {
           expect(options[2].getAttribute('role')).toEqual('option');
         }));
 
-        it('should set aria-selected on each option', fakeAsync(() => {
-          expect(options[0].getAttribute('aria-selected')).toEqual('false');
-          expect(options[1].getAttribute('aria-selected')).toEqual('false');
-          expect(options[2].getAttribute('aria-selected')).toEqual('false');
+        it('should set aria-selected on each option for single select',  fakeAsync(() => {
+          expect(options.every(option => !option.hasAttribute('aria-selected'))).toBe(true,
+            'Expected all unselected single-select options not to have aria-selected set.');
 
           options[1].click();
           fixture.detectChanges();
@@ -894,11 +892,44 @@ describe('MatSelect', () => {
           fixture.detectChanges();
           flush();
 
-          expect(options[0].getAttribute('aria-selected')).toEqual('false');
-          expect(options[1].getAttribute('aria-selected')).toEqual('true');
-          expect(options[2].getAttribute('aria-selected')).toEqual('false');
+          expect(options[1].getAttribute('aria-selected')).toEqual('true',
+            'Expected selected single-select option to have aria-selected="true".');
+          options.splice(1, 1);
+          expect(options.every(option => !option.hasAttribute('aria-selected'))).toBe(true,
+            'Expected all unselected single-select options not to have aria-selected set.');
         }));
 
+        it('should set aria-selected on each option for multi-select', fakeAsync(() => {
+          fixture.destroy();
+
+          const multiFixture = TestBed.createComponent(MultiSelect);
+          multiFixture.detectChanges();
+
+          trigger = multiFixture.debugElement.query(By.css('.mat-select-trigger')).nativeElement;
+          trigger.click();
+          multiFixture.detectChanges();
+
+          options = Array.from(overlayContainerElement.querySelectorAll('mat-option'));
+
+          expect(options.every(option => option.hasAttribute('aria-selected') &&
+            option.getAttribute('aria-selected') === 'false')).toBe(true,
+            'Expected all unselected multi-select options to have aria-selected="false".');
+
+          options[1].click();
+          multiFixture.detectChanges();
+
+          trigger.click();
+          multiFixture.detectChanges();
+          flush();
+
+          expect(options[1].getAttribute('aria-selected')).toEqual('true',
+            'Expected selected multi-select option to have aria-selected="true".');
+          options.splice(1, 1);
+          expect(options.every(option => option.hasAttribute('aria-selected') &&
+            option.getAttribute('aria-selected') === 'false')).toBe(true,
+            'Expected all unselected multi-select options to have aria-selected="false".');
+          }));
+
         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');

tools/public_api_guard/lib/core.d.ts

@@ -244,6 +244,7 @@ export declare class MatOption implements AfterViewChecked, OnDestroy {
     value: any;
     readonly viewValue: string;
     constructor(_element: ElementRef<HTMLElement>, _changeDetectorRef: ChangeDetectorRef, _parent: MatOptionParentComponent, group: MatOptgroup);
+    _getAriaSelected(): boolean | null;
     _getHostElement(): HTMLElement;
     _getTabIndex(): string;
     _handleKeydown(event: KeyboardEvent): void;