title: Angular: cdkVirtualFor not rendering new items
body: <p>I'm building a vertically scrolling calendar. I'm getting the initial days to load, but when new days are added to the list, they aren't being rendered. </p>
  
  <pre><code>&lt;cdk-virtual-scroll-viewport
    class="demo-viewport"
    [itemSize]="100"
    (onContentScrolled)="handleScrollChange($event)"
  &gt;
    &lt;calendar-day
      *cdkVirtualFor="let day of days; trackBy: trackByFn"
      [day]="day"
    &gt;&lt;/calendar-day&gt;
  &lt;/cdk-virtual-scroll-viewport&gt;
  &lt;button (click)="goToToday()"&gt;go&lt;/button&gt;
  </code></pre>
  
  <p>I have a service with a <code>BehaviorSubject</code> updating the days. I know the list of days is being updated, but the change doesn't seem to be detected.</p>
  
  <pre><code>  ngOnInit() {
      this._daysService.days$.subscribe(days =&gt; {
        this.days = days;
      })
      this.watchScroll();
      this.handleScrollingUp();
      this.handleScrollingDown();
    }
  </code></pre>
  
  <p>For more info, the StackBlitz repo is public <a href="https://stackblitz.com/edit/material-infinite-calendar" rel="noreferrer">https://stackblitz.com/edit/material-infinite-calendar</a></p>