fix(expansion-panel): focus lost if focused element is inside closing panel (#12692)

Currently when an expansion panel is closed, we make the content non-focusable using `visibility: hidden`, but that means that if the focused element was inside the panel, focus will be returned back to the body. These changes add a listener that will restore focus to the panel header, if the focused element is inside the panel when it is closed.

Author: crisbeto

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

@@ -83,7 +83,12 @@ export class MatExpansionPanelHeader implements OnDestroy {
     )
     .subscribe(() => this._changeDetectorRef.markForCheck());
 
-    _focusMonitor.monitor(_element.nativeElement);
+    // Avoids focus being lost if the panel contained the focused element and was closed.
+    panel.closed
+      .pipe(filter(() => panel._containsFocus()))
+      .subscribe(() => _focusMonitor.focusVia(_element.nativeElement, 'program'));
+
+      _focusMonitor.monitor(_element.nativeElement);
   }
 
   /** Height of the header while the panel is expanded. */

src/lib/expansion/expansion-panel.ts

@@ -11,19 +11,23 @@ import {CdkAccordionItem} from '@angular/cdk/accordion';
 import {coerceBooleanProperty} from '@angular/cdk/coercion';
 import {UniqueSelectionDispatcher} from '@angular/cdk/collections';
 import {TemplatePortal} from '@angular/cdk/portal';
+import {DOCUMENT} from '@angular/common';
 import {
   AfterContentInit,
   ChangeDetectionStrategy,
   ChangeDetectorRef,
   Component,
   ContentChild,
   Directive,
+  ElementRef,
+  Inject,
   Input,
   OnChanges,
   OnDestroy,
   Optional,
   SimpleChanges,
   SkipSelf,
+  ViewChild,
   ViewContainerRef,
   ViewEncapsulation,
 } from '@angular/core';
@@ -70,8 +74,13 @@ let uniqueId = 0;
     '[class.mat-expansion-panel-spacing]': '_hasSpacing()',
   }
 })
-export class MatExpansionPanel extends _CdkAccordionItem
-  implements AfterContentInit, OnChanges, OnDestroy {
+export class MatExpansionPanel extends CdkAccordionItem implements AfterContentInit, OnChanges,
+  OnDestroy {
+
+  // @breaking-change 8.0.0 Remove `| undefined` from here
+  // when the `_document` constructor param is required.
+  private _document: Document | undefined;
+
   /** Whether the toggle indicator should be hidden. */
   @Input()
   get hideToggle(): boolean {
@@ -91,6 +100,9 @@ export class MatExpansionPanel extends _CdkAccordionItem
   /** Content that will be rendered lazily. */
   @ContentChild(MatExpansionPanelContent) _lazyContent: MatExpansionPanelContent;
 
+  /** Element containing the panel's user-provided content. */
+  @ViewChild('body') _body: ElementRef<HTMLElement>;
+
   /** Portal holding the user's content. */
   _portal: TemplatePortal;
 
@@ -100,9 +112,11 @@ export class MatExpansionPanel extends _CdkAccordionItem
   constructor(@Optional() @SkipSelf() accordion: MatAccordion,
               _changeDetectorRef: ChangeDetectorRef,
               _uniqueSelectionDispatcher: UniqueSelectionDispatcher,
-              private _viewContainerRef: ViewContainerRef) {
+              private _viewContainerRef: ViewContainerRef,
+              @Inject(DOCUMENT) _document?: any) {
     super(accordion, _changeDetectorRef, _uniqueSelectionDispatcher);
     this.accordion = accordion;
+    this._document = _document;
   }
 
   /** Determines whether the expansion panel should have spacing between it and its siblings. */
@@ -158,6 +172,17 @@ export class MatExpansionPanel extends _CdkAccordionItem
       classList.remove(cssClass);
     }
   }
+
+  /** Checks whether the expansion panel's content contains the currently-focused element. */
+  _containsFocus(): boolean {
+    if (this._body && this._document) {
+      const focusedElement = this._document.activeElement;
+      const bodyElement = this._body.nativeElement;
+      return focusedElement === bodyElement || bodyElement.contains(focusedElement);
+    }
+
+    return false;
+  }
 }
 
 @Directive({

src/lib/expansion/expansion.spec.ts

@@ -160,6 +160,25 @@ describe('MatExpansionPanel', () => {
     expect(document.activeElement).not.toBe(button, 'Expected button to no longer be focusable.');
   }));
 
+  it('should restore focus to header if focused element is inside panel on close', fakeAsync(() => {
+    const fixture = TestBed.createComponent(PanelWithContent);
+    fixture.componentInstance.expanded = true;
+    fixture.detectChanges();
+    tick(250);
+
+    const button = fixture.debugElement.query(By.css('button')).nativeElement;
+    const header = fixture.debugElement.query(By.css('mat-expansion-panel-header')).nativeElement;
+
+    button.focus();
+    expect(document.activeElement).toBe(button, 'Expected button to start off focusable.');
+
+    fixture.componentInstance.expanded = false;
+    fixture.detectChanges();
+    tick(250);
+
+    expect(document.activeElement).toBe(header, 'Expected header to be focused.');
+  }));
+
   it('should not override the panel margin if it is not inside an accordion', fakeAsync(() => {
     let fixture = TestBed.createComponent(PanelWithCustomMargin);
     fixture.detectChanges();