fix(autocomplete): close panel using alt + up arrow (#9341)

[Based on the accessibility guidelines](https://www.w3.org/TR/wai-aria-practices-1.1/#textbox-keyboard-interaction), the autocomplete panel can be closed using the alt + up arrow keyboard combo.

src/lib/autocomplete/autocomplete-trigger.ts

@@ -129,8 +129,8 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
   /** The subscription for closing actions (some are bound to document). */
   private _closingActionsSubscription: Subscription;
 
-  /** Stream of escape keyboard events. */
-  private _escapeEventStream = new Subject<void>();
+  /** Stream of keyboard events that can close the panel. */
+  private _closeKeyEventStream = new Subject<void>();
 
   /** View -> model callback called when value changes */
   _onChange: (value: any) => void = () => {};
@@ -152,7 +152,7 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
 
   ngOnDestroy() {
     this._destroyPanel();
-    this._escapeEventStream.complete();
+    this._closeKeyEventStream.complete();
   }
 
   /** Whether or not the autocomplete panel is open. */
@@ -194,7 +194,7 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
     return merge(
       this.optionSelections,
       this.autocomplete._keyManager.tabOut.pipe(filter(() => this._panelOpen)),
-      this._escapeEventStream,
+      this._closeKeyEventStream,
       this._outsideClickStream,
       this._overlayRef ?
           this._overlayRef.detachments().pipe(filter(() => this._panelOpen)) :
@@ -289,9 +289,11 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
   _handleKeydown(event: KeyboardEvent): void {
     const keyCode = event.keyCode;
 
-    if (keyCode === ESCAPE && this.panelOpen) {
+    // Close when pressing ESCAPE or ALT + UP_ARROW, based on the a11y guidelines.
+    // See: https://www.w3.org/TR/wai-aria-practices-1.1/#textbox-keyboard-interaction
+    if (this.panelOpen && (keyCode === ESCAPE || (keyCode === UP_ARROW && event.altKey))) {
       this._resetActiveItem();
-      this._escapeEventStream.next();
+      this._closeKeyEventStream.next();
       event.stopPropagation();
     } else if (this.activeOption && keyCode === ENTER && this.panelOpen) {
       this.activeOption._selectViaInteraction();

src/lib/autocomplete/autocomplete.spec.ts

@@ -902,6 +902,25 @@ describe('MatAutocomplete', () => {
       expect(stopPropagationSpy).toHaveBeenCalled();
     }));
 
+    it('should close the panel when pressing ALT + UP_ARROW', fakeAsync(() => {
+      const trigger = fixture.componentInstance.trigger;
+      const upArrowEvent = createKeyboardEvent('keydown', UP_ARROW);
+      Object.defineProperty(upArrowEvent, 'altKey', {get: () => true});
+
+      input.focus();
+      flush();
+      fixture.detectChanges();
+
+      expect(document.activeElement).toBe(input, 'Expected input to be focused.');
+      expect(trigger.panelOpen).toBe(true, 'Expected panel to be open.');
+
+      trigger._handleKeydown(upArrowEvent);
+      fixture.detectChanges();
+
+      expect(document.activeElement).toBe(input, 'Expected input to continue to be focused.');
+      expect(trigger.panelOpen).toBe(false, 'Expected panel to be closed.');
+    }));
+
     it('should close the panel when tabbing away from a trigger without results', fakeAsync(() => {
       fixture.componentInstance.states = [];
       fixture.componentInstance.filteredStates = [];