is-4.ts

@Component({
  selector: 'infinite-scroll',
  template: `<ng-content></ng-content><div #anchor></div>`
})
export class InfiniteScrollComponent implements OnInit, OnDestroy {
  ...

  ngOnInit() {
    const options = {
      root: this.isHostScrollable() ? this.host.nativeElement : null,
      ...this.options
    };

    this.observer = new IntersectionObserver(([entry]) => {
      entry.isIntersecting && this.scrolled.emit();
    }, options);

    this.observer.observe(this.anchor.nativeElement);
  }

  private isHostScrollable() {
    const style = window.getComputedStyle(this.element);

    return style.getPropertyValue('overflow') === 'auto' ||
      style.getPropertyValue('overflow-y') === 'scroll';
  }
}