fix(autocomplete): don't handle enter events with modifier keys

Doesn't handle `ENTER` key presses and doesn't prevent their default action, if they have a modifier, in order to avoid intefering with any OS-level shortcuts.

Author: crisbeto

src/material/autocomplete/autocomplete-trigger.ts

@@ -7,7 +7,7 @@
  */
 import {Directionality} from '@angular/cdk/bidi';
 import {BooleanInput, coerceBooleanProperty} from '@angular/cdk/coercion';
-import {DOWN_ARROW, ENTER, ESCAPE, TAB, UP_ARROW} from '@angular/cdk/keycodes';
+import {DOWN_ARROW, ENTER, ESCAPE, TAB, UP_ARROW, hasModifierKey} from '@angular/cdk/keycodes';
 import {
   FlexibleConnectedPositionStrategy,
   Overlay,
@@ -406,7 +406,7 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, AfterViewIn
       event.preventDefault();
     }
 
-    if (this.activeOption && keyCode === ENTER && this.panelOpen) {
+    if (this.activeOption && keyCode === ENTER && this.panelOpen && !hasModifierKey(event)) {
       this.activeOption._selectViaInteraction();
       this._resetActiveItem();
       event.preventDefault();

src/material/autocomplete/autocomplete.spec.ts

@@ -988,6 +988,26 @@ describe('MatAutocomplete', () => {
       expect(ENTER_EVENT.defaultPrevented).toBe(false, 'Default action should not be prevented.');
     });
 
+    it('should not interfere with the ENTER key when pressing a modifier', fakeAsync(() => {
+      const trigger = fixture.componentInstance.trigger;
+
+      expect(input.value).toBeFalsy('Expected input to start off blank.');
+      expect(trigger.panelOpen).toBe(true, 'Expected panel to start off open.');
+
+      fixture.componentInstance.trigger._handleKeydown(DOWN_ARROW_EVENT);
+      flush();
+      fixture.detectChanges();
+
+      Object.defineProperty(ENTER_EVENT, 'altKey', {get: () => true});
+      fixture.componentInstance.trigger._handleKeydown(ENTER_EVENT);
+      fixture.detectChanges();
+
+      expect(trigger.panelOpen).toBe(true, 'Expected panel to remain open.');
+      expect(input.value).toBeFalsy('Expected input to remain blank.');
+      expect(ENTER_EVENT.defaultPrevented)
+          .toBe(false, 'Expected the default ENTER action not to have been prevented.');
+    }));
+
     it('should fill the text field, not select an option, when SPACE is entered', () => {
       typeInElement(input, 'New');
       fixture.detectChanges();