fix(list-key-manager): prevent the default keyboard actions

Calls preventDefault when handling keyboard events in the ListKeyManager. This avoids the user scrolling the page when using the arrow keys to navigate.

Relates to #1999.

Author: crisbeto

src/lib/core/a11y/list-key-manager.spec.ts

@@ -15,15 +15,23 @@ class FakeQueryList<T> extends QueryList<T> {
   }
 }
 
-const DOWN_ARROW_EVENT = { keyCode: DOWN_ARROW } as KeyboardEvent;
-const UP_ARROW_EVENT = { keyCode: UP_ARROW } as KeyboardEvent;
-const TAB_EVENT = { keyCode: TAB } as KeyboardEvent;
-const HOME_EVENT = { keyCode: HOME } as KeyboardEvent;
-const END_EVENT = { keyCode: END } as KeyboardEvent;
+class FakeEvent {
+  defaultPrevented: boolean = false;
+  constructor(public keyCode: number) {}
+  preventDefault() {
+    this.defaultPrevented = true;
+  }
+}
+
 
 describe('ListKeyManager', () => {
   let keyManager: ListKeyManager;
   let itemList: FakeQueryList<FakeFocusable>;
+  let DOWN_ARROW_EVENT: KeyboardEvent;
+  let UP_ARROW_EVENT: KeyboardEvent;
+  let TAB_EVENT: KeyboardEvent;
+  let HOME_EVENT: KeyboardEvent;
+  let END_EVENT: KeyboardEvent;
 
   beforeEach(() => {
     itemList = new FakeQueryList<FakeFocusable>();
@@ -35,6 +43,12 @@ describe('ListKeyManager', () => {
 
     keyManager = new ListKeyManager(itemList);
 
+    DOWN_ARROW_EVENT = new FakeEvent(DOWN_ARROW) as KeyboardEvent;
+    UP_ARROW_EVENT = new FakeEvent(UP_ARROW) as KeyboardEvent;
+    TAB_EVENT = new FakeEvent(TAB) as KeyboardEvent;
+    HOME_EVENT = new FakeEvent(HOME) as KeyboardEvent;
+    END_EVENT = new FakeEvent(END) as KeyboardEvent;
+
     // first item is already focused
     keyManager.focusFirstItem();
 
@@ -166,6 +180,14 @@ describe('ListKeyManager', () => {
       expect(tabOutEmitted).toBe(true);
     });
 
+    it('should prevent the default keyboard action', () => {
+      expect(DOWN_ARROW_EVENT.defaultPrevented).toBe(false);
+
+      keyManager.onKeydown(DOWN_ARROW_EVENT);
+
+      expect(DOWN_ARROW_EVENT.defaultPrevented).toBe(true);
+    });
+
   });
 
   describe('programmatic focus', () => {

src/lib/core/a11y/list-key-manager.ts

@@ -40,6 +40,8 @@ export class ListKeyManager {
 
   /** Sets the focus properly depending on the key event passed in. */
   onKeydown(event: KeyboardEvent): void {
+    event.preventDefault();
+
     switch (event.keyCode) {
       case DOWN_ARROW:
         this.focusNextItem();