fix(material/autocomplete): don't block default arrow keys when using modifiers

src/material-experimental/mdc-autocomplete/autocomplete.spec.ts

@@ -1470,6 +1470,20 @@ describe('MDC-based MatAutocomplete', () => {
 
       expect(!!trigger.activeOption).withContext('Expected no active options.').toBe(false);
     }));
+
+    it('should not prevent the default action when a modifier key is pressed', () => {
+      ['metaKey', 'ctrlKey', 'altKey', 'shiftKey'].forEach(name => {
+        const event = createKeyboardEvent('keydown', DOWN_ARROW);
+        Object.defineProperty(event, name, {get: () => true});
+
+        fixture.componentInstance.trigger._handleKeydown(event);
+        fixture.detectChanges();
+
+        expect(event.defaultPrevented)
+          .withContext(`Expected autocompete not to block ${name} key`)
+          .toBe(false);
+      });
+    });
   });
 
   describe('option groups', () => {

src/material/autocomplete/autocomplete-trigger.ts

@@ -390,24 +390,25 @@ export abstract class _MatAutocompleteTriggerBase
 
   _handleKeydown(event: KeyboardEvent): void {
     const keyCode = event.keyCode;
+    const hasModifier = hasModifierKey(event);
 
     // Prevent the default action on all escape key presses. This is here primarily to bring IE
     // in line with other browsers. By default, pressing escape on IE will cause it to revert
     // the input value to the one that it had on focus, however it won't dispatch any events
     // which means that the model value will be out of sync with the view.
-    if (keyCode === ESCAPE && !hasModifierKey(event)) {
+    if (keyCode === ESCAPE && !hasModifier) {
       event.preventDefault();
     }
 
-    if (this.activeOption && keyCode === ENTER && this.panelOpen && !hasModifierKey(event)) {
+    if (this.activeOption && keyCode === ENTER && this.panelOpen && !hasModifier) {
       this.activeOption._selectViaInteraction();
       this._resetActiveItem();
       event.preventDefault();
     } else if (this.autocomplete) {
       const prevActiveItem = this.autocomplete._keyManager.activeItem;
       const isArrowKey = keyCode === UP_ARROW || keyCode === DOWN_ARROW;
 
-      if (this.panelOpen || keyCode === TAB) {
+      if (keyCode === TAB || (isArrowKey && !hasModifier && this.panelOpen)) {
         this.autocomplete._keyManager.onKeydown(event);
       } else if (isArrowKey && this._canOpen()) {
         this.openPanel();

src/material/autocomplete/autocomplete.spec.ts

@@ -1453,6 +1453,20 @@ describe('MatAutocomplete', () => {
 
       expect(!!trigger.activeOption).withContext('Expected no active options.').toBe(false);
     }));
+
+    it('should not prevent the default action when a modifier key is pressed', () => {
+      ['metaKey', 'ctrlKey', 'altKey', 'shiftKey'].forEach(name => {
+        const event = createKeyboardEvent('keydown', DOWN_ARROW);
+        Object.defineProperty(event, name, {get: () => true});
+
+        fixture.componentInstance.trigger._handleKeydown(event);
+        fixture.detectChanges();
+
+        expect(event.defaultPrevented)
+          .withContext(`Expected autocompete not to block ${name} key`)
+          .toBe(false);
+      });
+    });
   });
 
   describe('option groups', () => {