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

Currently we hijack all up/down arrow key events, however this interferes with keyboard shortcuts such as shift + up arrow for marking all of the text. These changes stop intercepting the arrow keys, if they're used with a modifier.

These changes also fix an issue where all the mocked out key events had the `metaKey` set to `true` on some browsers.

src/cdk/testing/event-objects.ts

@@ -51,11 +51,14 @@ export function createTouchEvent(type: string, pageX = 0, pageY = 0) {
 /** Dispatches a keydown event from an element. */
 export function createKeyboardEvent(type: string, keyCode: number, target?: Element, key?: string) {
   let event = document.createEvent('KeyboardEvent') as any;
-  // Firefox does not support `initKeyboardEvent`, but supports `initKeyEvent`.
-  let initEventFn = (event.initKeyEvent || event.initKeyboardEvent).bind(event);
   let originalPreventDefault = event.preventDefault;
 
-  initEventFn(type, true, true, window, 0, 0, 0, 0, 0, keyCode);
+  // Firefox does not support `initKeyboardEvent`, but supports `initKeyEvent`.
+  if (event.initKeyEvent) {
+    event.initKeyEvent(type, true, true, window, 0, 0, 0, 0, 0, keyCode);
+  } else {
+    event.initKeyboardEvent(type, true, true, window, 0, key, 0, '', false);
+  }
 
   // Webkit Browsers don't set the keyCode when calling the init function.
   // See related bug https://bugs.webkit.org/show_bug.cgi?id=16735

src/lib/autocomplete/autocomplete-trigger.ts

@@ -336,15 +336,16 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
       event.preventDefault();
     } else {
       const prevActiveItem = this.autocomplete._keyManager.activeItem;
-      const isArrowKey = keyCode === UP_ARROW || keyCode === DOWN_ARROW;
+      const isVerticalArrowKey = keyCode === UP_ARROW || keyCode === DOWN_ARROW;
+      const hasModifier = event.ctrlKey || event.altKey || event.metaKey || event.shiftKey;
 
-      if (this.panelOpen || keyCode === TAB) {
+      if ((isVerticalArrowKey && !hasModifier) || keyCode === TAB) {
         this.autocomplete._keyManager.onKeydown(event);
-      } else if (isArrowKey && this._canOpen()) {
+      } else if (isVerticalArrowKey && this._canOpen()) {
         this.openPanel();
       }
 
-      if (isArrowKey || this.autocomplete._keyManager.activeItem !== prevActiveItem) {
+      if (isVerticalArrowKey || this.autocomplete._keyManager.activeItem !== prevActiveItem) {
         this._scrollToOption();
       }
     }

src/lib/autocomplete/autocomplete.spec.ts

@@ -1148,6 +1148,18 @@ describe('MatAutocomplete', () => {
       expect(!!trigger.activeOption).toBe(false, 'Expected no active options.');
     }));
 
+    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).toBe(false, `Expected autocompete not to block ${name} key`);
+      });
+    });
+
   });
 
   describe('option groups', () => {