virtual-scroll: switch back to markForCheck since detectChanges (#12329)

breaks viewports that are inside an `OnPush` component.

Fixes issue introduced by #12158

src/cdk-experimental/scrolling/virtual-scroll-viewport.spec.ts

@@ -101,9 +101,10 @@ describe('CdkVirtualScrollViewport', () => {
 
     it('should set total content size', fakeAsync(() => {
       finishInit(fixture);
+
       viewport.setTotalContentSize(10000);
-      fixture.detectChanges();
       flush();
+      fixture.detectChanges();
 
       expect(viewport.elementRef.nativeElement.scrollHeight).toBe(10000);
     }));

src/cdk-experimental/scrolling/virtual-scroll-viewport.ts

@@ -21,7 +21,7 @@ import {
   ViewEncapsulation,
 } from '@angular/core';
 import {animationFrameScheduler, fromEvent, Observable, Subject} from 'rxjs';
-import {sampleTime, take, takeUntil} from 'rxjs/operators';
+import {sampleTime, takeUntil} from 'rxjs/operators';
 import {CdkVirtualForOf} from './virtual-for-of';
 import {VIRTUAL_SCROLL_STRATEGY, VirtualScrollStrategy} from './virtual-scroll-strategy';
 
@@ -301,11 +301,7 @@ export class CdkVirtualScrollViewport implements OnInit, OnDestroy {
     if (!this._isChangeDetectionPending) {
       this._isChangeDetectionPending = true;
       this._ngZone.runOutsideAngular(() => Promise.resolve().then(() => {
-        if (this._ngZone.isStable) {
-           this._doChangeDetection();
-        } else {
-          this._ngZone.onStable.pipe(take(1)).subscribe(() => this._doChangeDetection());
-        }
+        this._doChangeDetection();
       }));
     }
   }
@@ -314,8 +310,10 @@ export class CdkVirtualScrollViewport implements OnInit, OnDestroy {
   private _doChangeDetection() {
     this._isChangeDetectionPending = false;
 
-    // Apply changes to Angular bindings.
-    this._ngZone.run(() => this._changeDetectorRef.detectChanges());
+    // Apply changes to Angular bindings. Note: We must call `markForCheck` to run change detection
+    // from the root, since the repeated items are content projected in. Calling `detectChanges`
+    // instead does not properly check the projected content.
+    this._ngZone.run(() => this._changeDetectorRef.markForCheck());
     // Apply the content transform. The transform can't be set via an Angular binding because
     // bypassSecurityTrustStyle is banned in Google. However the value is safe, it's composed of
     // string literals, a variable that can only be 'X' or 'Y', and user input that is run through
@@ -330,9 +328,10 @@ export class CdkVirtualScrollViewport implements OnInit, OnDestroy {
       }
     }
 
-    for (const fn of this._runAfterChangeDetection) {
+    const runAfterChangeDetection = this._runAfterChangeDetection;
+    this._runAfterChangeDetection = [];
+    for (const fn of runAfterChangeDetection) {
       fn();
     }
-    this._runAfterChangeDetection = [];
   }
 }

src/demo-app/virtual-scroll/virtual-scroll-demo.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Component, ViewEncapsulation} from '@angular/core';
+import {ChangeDetectionStrategy, Component, ViewEncapsulation} from '@angular/core';
 import {BehaviorSubject} from 'rxjs';
 
 
@@ -22,6 +22,7 @@ type State = {
   templateUrl: 'virtual-scroll-demo.html',
   styleUrls: ['virtual-scroll-demo.css'],
   encapsulation: ViewEncapsulation.None,
+  changeDetection: ChangeDetectionStrategy.OnPush,
 })
 export class VirtualScrollDemo {
   fixedSizeData = Array(10000).fill(50);