fix(autocomplete): adding aria-activedescendant while closed if autoActiveFirstOption is enabled

Fixes the autocomplete trigger setting its `aria-activedescendant` attribute while the panel is closed and the options aren't in the DOM, if the `autoActiveFirstOption` is enabled.

Fixes #14453.

Author: crisbeto

src/lib/autocomplete/autocomplete-trigger.ts

@@ -102,7 +102,7 @@ export function getMatAutocompleteMissingPanelError(): Error {
     '[attr.autocomplete]': 'autocompleteAttribute',
     '[attr.role]': 'autocompleteDisabled ? null : "combobox"',
     '[attr.aria-autocomplete]': 'autocompleteDisabled ? null : "list"',
-    '[attr.aria-activedescendant]': 'activeOption?.id',
+    '[attr.aria-activedescendant]': '(panelOpen && activeOption) ? activeOption.id : null',
     '[attr.aria-expanded]': 'autocompleteDisabled ? null : panelOpen.toString()',
     '[attr.aria-owns]': '(autocompleteDisabled || !panelOpen) ? null : autocomplete?.id',
     // Note: we use `focusin`, as opposed to `focus`, in order to open the panel

src/lib/autocomplete/autocomplete.spec.ts

@@ -1658,6 +1658,29 @@ describe('MatAutocomplete', () => {
 
         expect(overlayContainerElement.querySelectorAll('mat-option')[0].classList)
             .toContain('mat-active', 'Expected first option to be highlighted.');
+    }));
+
+    it('should remove aria-activedescendant when panel is closed with autoActiveFirstOption',
+      fakeAsync(() => {
+        const input: HTMLElement = fixture.nativeElement.querySelector('input');
+
+        expect(input.hasAttribute('aria-activedescendant'))
+            .toBe(false, 'Expected no active descendant on init.');
+
+        fixture.componentInstance.trigger.autocomplete.autoActiveFirstOption = true;
+        fixture.componentInstance.trigger.openPanel();
+        fixture.detectChanges();
+        zone.simulateZoneExit();
+        fixture.detectChanges();
+
+        expect(input.getAttribute('aria-activedescendant'))
+            .toBeTruthy('Expected active descendant while open.');
+
+        fixture.componentInstance.trigger.closePanel();
+        fixture.detectChanges();
+
+        expect(input.hasAttribute('aria-activedescendant'))
+            .toBe(false, 'Expected no active descendant when closed.');
       }));
 
     it('should be able to configure preselecting the first option globally', fakeAsync(() => {