fix(expansion-panel): improved accessibility labelling and keyboard default action not being prevented (#9174)

Adds the following improvements to the expansion accessibility, based on the [accessible accordion guidelines](https://www.w3.org/TR/wai-aria-practices-1.1/examples/accordion/accordion.html):
* Adds the `region` role to the content element.
* Adds `aria-labelledby` pointing to the header.
* Fixes the page being scrolled down when a panel is opened using space. The issue was that we were preventing the default action during `keydown` which is too late in the lifecycle.

Also adds a couple of extra tests for the keyboard controls.

src/lib/expansion/expansion-panel-header.ts

@@ -46,13 +46,14 @@ import {matExpansionAnimations} from './expansion-animations';
   host: {
     'class': 'mat-expansion-panel-header',
     'role': 'button',
+    '[attr.id]': 'panel._headerId',
     '[attr.tabindex]': 'panel.disabled ? -1 : 0',
     '[attr.aria-controls]': '_getPanelId()',
     '[attr.aria-expanded]': '_isExpanded()',
     '[attr.aria-disabled]': 'panel.disabled',
     '[class.mat-expanded]': '_isExpanded()',
     '(click)': '_toggle()',
-    '(keyup)': '_keyup($event)',
+    '(keydown)': '_keydown($event)',
     '[@expansionHeight]': `{
         value: _getExpandedState(),
         params: {
@@ -116,8 +117,8 @@ export class MatExpansionPanelHeader implements OnDestroy {
     return !this.panel.hideToggle && !this.panel.disabled;
   }
 
-  /** Handle keyup event calling to toggle() if appropriate. */
-  _keyup(event: KeyboardEvent) {
+  /** Handle keydown event calling to toggle() if appropriate. */
+  _keydown(event: KeyboardEvent) {
     switch (event.keyCode) {
       // Toggle for space and enter keys.
       case SPACE:

src/lib/expansion/expansion-panel.html

@@ -1,8 +1,10 @@
 <ng-content select="mat-expansion-panel-header"></ng-content>
 <div class="mat-expansion-panel-content"
+     role="region"
      [class.mat-expanded]="expanded"
      [@bodyExpansion]="_getExpandedState()"
-     [id]="id">
+     [id]="id"
+     [attr.aria-labelledby]="_headerId">
   <div class="mat-expansion-panel-body">
     <ng-content></ng-content>
     <ng-template [cdkPortalOutlet]="_portal"></ng-template>

src/lib/expansion/expansion-panel.ts

@@ -57,6 +57,9 @@ export const _MatExpansionPanelMixinBase = mixinDisabled(MatExpansionPanelBase);
 /** MatExpansionPanel's states. */
 export type MatExpansionPanelState = 'expanded' | 'collapsed';
 
+/** Counter for generating unique element ids. */
+let uniqueId = 0;
+
 /**
  * <mat-expansion-panel>
  *
@@ -107,6 +110,9 @@ export class MatExpansionPanel extends _MatExpansionPanelMixinBase
   /** Portal holding the user's content. */
   _portal: TemplatePortal;
 
+  /** ID for the associated header element. Used for a11y labelling. */
+  _headerId = `mat-expansion-panel-header-${uniqueId++}`;
+
   constructor(@Optional() @Host() accordion: MatAccordion,
               _changeDetectorRef: ChangeDetectorRef,
               _uniqueSelectionDispatcher: UniqueSelectionDispatcher,

src/lib/expansion/expansion.spec.ts

@@ -3,6 +3,8 @@ import {Component, ViewChild} from '@angular/core';
 import {By} from '@angular/platform-browser';
 import {NoopAnimationsModule} from '@angular/platform-browser/animations';
 import {MatExpansionModule, MatExpansionPanel} from './index';
+import {SPACE, ENTER} from '@angular/cdk/keycodes';
+import {dispatchKeyboardEvent} from '@angular/cdk/testing';
 
 
 describe('MatExpansionPanel', () => {
@@ -72,7 +74,7 @@ describe('MatExpansionPanel', () => {
     expect(fixture.componentInstance.closeCallback).toHaveBeenCalled();
   });
 
-  it('creates a unique panel id for each panel', () => {
+  it('should create a unique panel id for each panel', () => {
     const fixtureOne = TestBed.createComponent(PanelWithContent);
     const headerElOne = fixtureOne.nativeElement.querySelector('.mat-expansion-panel-header');
     const fixtureTwo = TestBed.createComponent(PanelWithContent);
@@ -85,9 +87,58 @@ describe('MatExpansionPanel', () => {
     expect(panelIdOne).not.toBe(panelIdTwo);
   });
 
+  it('should set `aria-labelledby` of the content to the header id', () => {
+    const fixture = TestBed.createComponent(PanelWithContent);
+    const headerEl = fixture.nativeElement.querySelector('.mat-expansion-panel-header');
+    const contentEl = fixture.nativeElement.querySelector('.mat-expansion-panel-content');
+
+    fixture.detectChanges();
+
+    const headerId = headerEl.getAttribute('id');
+    const contentLabel = contentEl.getAttribute('aria-labelledby');
+
+    expect(headerId).toBeTruthy();
+    expect(contentLabel).toBeTruthy();
+    expect(headerId).toBe(contentLabel);
+  });
+
+  it('should set the proper role on the content element', () => {
+    const fixture = TestBed.createComponent(PanelWithContent);
+    const contentEl = fixture.nativeElement.querySelector('.mat-expansion-panel-content');
+
+    expect(contentEl.getAttribute('role')).toBe('region');
+  });
+
+  it('should toggle the panel when pressing SPACE on the header', () => {
+    const fixture = TestBed.createComponent(PanelWithContent);
+    const headerEl = fixture.nativeElement.querySelector('.mat-expansion-panel-header');
+
+    spyOn(fixture.componentInstance.panel, 'toggle');
+
+    const event = dispatchKeyboardEvent(headerEl, 'keydown', SPACE);
+
+    fixture.detectChanges();
+
+    expect(fixture.componentInstance.panel.toggle).toHaveBeenCalled();
+    expect(event.defaultPrevented).toBe(true);
+  });
+
+  it('should toggle the panel when pressing ENTER on the header', () => {
+    const fixture = TestBed.createComponent(PanelWithContent);
+    const headerEl = fixture.nativeElement.querySelector('.mat-expansion-panel-header');
+
+    spyOn(fixture.componentInstance.panel, 'toggle');
+
+    const event = dispatchKeyboardEvent(headerEl, 'keydown', ENTER);
+
+    fixture.detectChanges();
+
+    expect(fixture.componentInstance.panel.toggle).toHaveBeenCalled();
+    expect(event.defaultPrevented).toBe(true);
+  });
+
   it('should not be able to focus content while closed', fakeAsync(() => {
     const fixture = TestBed.createComponent(PanelWithContent);
-
     fixture.componentInstance.expanded = true;
     fixture.detectChanges();
     tick(250);