fix(expansion-panel): picking up lazy content from child component

We use a `ContentChild` to determine what lazy content to render inside an expansion panel. When the lazy content is nested further down inside another expansion panel, all ancestor panels up the tree will pick up the lowest lazy content from the lowest level. These changes add a check to ensure that the lazy content is rendered out by the closest panel.

Fixes angular#14365.

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

@@ -0,0 +1,25 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {InjectionToken} from '@angular/core';
+import {CdkAccordionItem} from '@angular/cdk/accordion';
+
+/**
+ * Base interface for a `MatExpansionPanel`.
+ * @docs-private
+ */
+export interface MatExpansionPanelBase extends CdkAccordionItem {
+  /** Whether the toggle indicator should be hidden. */
+  hideToggle: boolean;
+}
+
+/**
+ * Token used to provide a `MatExpansionPanel` to `MatExpansionPanelContent`.
+ * Used to avoid circular imports between `MatExpansionPanel` and `MatExpansionPanelContent`.
+ */
+export const MAT_EXPANSION_PANEL = new InjectionToken<MatExpansionPanelBase>('MAT_EXPANSION_PANEL');

src/material/expansion/expansion-panel-content.ts

@@ -6,7 +6,8 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Directive, TemplateRef} from '@angular/core';
+import {Directive, TemplateRef, Inject, Optional} from '@angular/core';
+import {MAT_EXPANSION_PANEL, MatExpansionPanelBase} from './expansion-panel-base';
 
 /**
  * Expansion panel content that will be rendered lazily
@@ -16,5 +17,7 @@ import {Directive, TemplateRef} from '@angular/core';
   selector: 'ng-template[matExpansionPanelContent]'
 })
 export class MatExpansionPanelContent {
-  constructor(public _template: TemplateRef<any>) {}
+  constructor(
+    public _template: TemplateRef<any>,
+    @Inject(MAT_EXPANSION_PANEL) @Optional() public _expansionPanel?: MatExpansionPanelBase) {}
 }

src/material/expansion/expansion-panel.ts

@@ -40,6 +40,7 @@ import {filter, startWith, take, distinctUntilChanged} from 'rxjs/operators';
 import {matExpansionAnimations} from './expansion-animations';
 import {MatExpansionPanelContent} from './expansion-panel-content';
 import {MAT_ACCORDION, MatAccordionBase} from './accordion-base';
+import {MAT_EXPANSION_PANEL} from './expansion-panel-base';
 
 /** MatExpansionPanel's states. */
 export type MatExpansionPanelState = 'expanded' | 'collapsed';
@@ -90,6 +91,7 @@ export const MAT_EXPANSION_PANEL_DEFAULT_OPTIONS =
     // Provide MatAccordion as undefined to prevent nested expansion panels from registering
     // to the same accordion.
     {provide: MAT_ACCORDION, useValue: undefined},
+    {provide: MAT_EXPANSION_PANEL, useExisting: MatExpansionPanel}
   ],
   host: {
     'class': 'mat-expansion-panel',
@@ -188,7 +190,7 @@ export class MatExpansionPanel extends CdkAccordionItem implements AfterContentI
   }
 
   ngAfterContentInit() {
-    if (this._lazyContent) {
+    if (this._lazyContent && this._lazyContent._expansionPanel === this) {
       // Render the content as soon as the panel becomes open.
       this.opened.pipe(
         startWith(null!),

src/material/expansion/expansion.spec.ts

@@ -25,6 +25,7 @@ describe('MatExpansionPanel', () => {
         PanelWithCustomMargin,
         LazyPanelWithContent,
         LazyPanelOpenOnLoad,
+        NestedLazyPanelWithContent,
         PanelWithTwoWayBinding,
       ],
     });
@@ -70,6 +71,28 @@ describe('MatExpansionPanel', () => {
         .toContain('Some content', 'Expected content to be rendered.');
   }));
 
+  it('should not render lazy content from a child panel inside the parent', fakeAsync(() => {
+    const fixture = TestBed.createComponent(NestedLazyPanelWithContent);
+    fixture.componentInstance.parentExpanded = true;
+    fixture.detectChanges();
+
+    const parentContent: HTMLElement =
+        fixture.nativeElement.querySelector('.parent-panel .mat-expansion-panel-content');
+    const childContent: HTMLElement =
+        fixture.nativeElement.querySelector('.child-panel .mat-expansion-panel-content');
+
+    expect(parentContent.textContent!.trim())
+        .toBe('Parent content', 'Expected only parent content to be rendered.');
+    expect(childContent.textContent!.trim())
+        .toBe('', 'Expected child content element to be empty.');
+
+    fixture.componentInstance.childExpanded = true;
+    fixture.detectChanges();
+
+    expect(childContent.textContent!.trim())
+        .toBe('Child content', 'Expected child content element to be rendered.');
+  }));
+
   it('emit correct events for change in panel expanded state', () => {
     const fixture = TestBed.createComponent(PanelWithContent);
     fixture.componentInstance.expanded = true;
@@ -490,3 +513,21 @@ class LazyPanelOpenOnLoad {}
 class PanelWithTwoWayBinding {
   expanded = false;
 }
+
+
+@Component({
+  template: `
+    <mat-expansion-panel class="parent-panel" [expanded]="parentExpanded">
+      Parent content
+
+      <mat-expansion-panel class="child-panel" [expanded]="childExpanded">
+        <ng-template matExpansionPanelContent>Child content</ng-template>
+      </mat-expansion-panel>
+    </mat-expansion-panel>
+  `
+})
+class NestedLazyPanelWithContent {
+  parentExpanded = false;
+  childExpanded = false;
+}
+