Alternative to @HostListener and scroll output on the element I would suggest using fromEvent from RxJS since you can chain it with filter() and distinctUntilChanges() and can easily skip flood of potentially redundant events (and change detections).
Here is a simple example:
// {static: true} can be omitted if you don't need this element/listener in ngOnInit
@ViewChild('elementId', {static: true}) el: ElementRef;
// ...
fromEvent(this.el.nativeElement, 'scroll')
.pipe(
// Is elementId scrolled for more than 50 from top?
map((e: Event) => (e.srcElement as Element).scrollTop > 50),
// Dispatch change only if result from map above is different from previous result
distinctUntilChanged());