fix(datepicker): retain focus on navigation links (#3381)

This fixes an issue on some clients due to browser being inconsistent on
handling focus on the button clicks. After the change the navigation
button within the datepicker will retain focus to allow subsequent
keyboard based navigation within datepicker instance.

Fixes: #2780

src/datepicker/datepicker-navigation.spec.ts

@@ -92,17 +92,35 @@ describe('ngb-datepicker-navigation', () => {
   it('should send navigation events', () => {
     const fixture =
         createTestComponent(`<ngb-datepicker-navigation (navigate)="onNavigate($event)"></ngb-datepicker-navigation>`);
-
-    const links = getNavigationLinks(fixture.nativeElement);
+    const[previousButton, nextButton] = getNavigationLinks(fixture.nativeElement);
+    const previousButtonSpan = previousButton.querySelector<HTMLElement>('span');
+    const nextButtonSpan = nextButton.querySelector<HTMLElement>('span');
     spyOn(fixture.componentInstance, 'onNavigate');
 
     // prev
-    links[0].click();
+    previousButton.click();
+    expect(fixture.componentInstance.onNavigate).toHaveBeenCalledWith(NavigationEvent.PREV);
+    previousButtonSpan.click();
     expect(fixture.componentInstance.onNavigate).toHaveBeenCalledWith(NavigationEvent.PREV);
 
     // next
-    links[1].click();
+    nextButton.click();
     expect(fixture.componentInstance.onNavigate).toHaveBeenCalledWith(NavigationEvent.NEXT);
+    nextButtonSpan.click();
+    expect(fixture.componentInstance.onNavigate).toHaveBeenCalledWith(NavigationEvent.NEXT);
+  });
+
+  it('should retain focus on the navigation links after click', () => {
+    const fixture = createTestComponent(`<ngb-datepicker-navigation></ngb-datepicker-navigation>`);
+    const[previousButton, nextButton] = getNavigationLinks(fixture.nativeElement);
+
+    // prev
+    previousButton.click();
+    expect(document.activeElement).toBe(previousButton);
+
+    // next
+    nextButton.click();
+    expect(document.activeElement).toBe(nextButton);
   });
 
   it('should have buttons of type button', () => {

src/datepicker/datepicker-navigation.ts

@@ -10,7 +10,7 @@ import {NgbDatepickerI18n} from './datepicker-i18n';
   styleUrls: ['./datepicker-navigation.scss'],
   template: `
     <div class="ngb-dp-arrow">
-      <button type="button" class="btn btn-link ngb-dp-arrow-btn" (click)="navigate.emit(navigation.PREV)" [disabled]="prevDisabled"
+      <button type="button" class="btn btn-link ngb-dp-arrow-btn" (click)="onClickPrev($event)" [disabled]="prevDisabled"
               i18n-aria-label="@@ngb.datepicker.previous-month" aria-label="Previous month"
               i18n-title="@@ngb.datepicker.previous-month" title="Previous month">
         <span class="ngb-dp-navigation-chevron"></span>
@@ -32,7 +32,7 @@ import {NgbDatepickerI18n} from './datepicker-i18n';
       <div class="ngb-dp-arrow" *ngIf="i !== months.length - 1"></div>
     </ng-template>
     <div class="ngb-dp-arrow right">
-      <button type="button" class="btn btn-link ngb-dp-arrow-btn" (click)="navigate.emit(navigation.NEXT)" [disabled]="nextDisabled"
+      <button type="button" class="btn btn-link ngb-dp-arrow-btn" (click)="onClickNext($event)" [disabled]="nextDisabled"
               i18n-aria-label="@@ngb.datepicker.next-month" aria-label="Next month"
               i18n-title="@@ngb.datepicker.next-month" title="Next month">
         <span class="ngb-dp-navigation-chevron"></span>
@@ -55,4 +55,14 @@ export class NgbDatepickerNavigation {
   @Output() select = new EventEmitter<NgbDate>();
 
   constructor(public i18n: NgbDatepickerI18n) {}
+
+  onClickPrev(event: MouseEvent) {
+    (event.currentTarget as HTMLElement).focus();
+    this.navigate.emit(this.navigation.PREV);
+  }
+
+  onClickNext(event: MouseEvent) {
+    (event.currentTarget as HTMLElement).focus();
+    this.navigate.emit(this.navigation.NEXT);
+  }
 }