Draggable mini scrolling video with RxJS

draggableMiniVideo.js

const video = document.getElementById('video');
const anchor = documant.getElementById('anchor');

const scroll$ = Rx.Observable.fromEvent(document, 'scroll');

scroll$
  .map(event => anchor.getBoundingClientRect().bottom < 0)
  .subscribe(isScrollOverAnchor => {
    if (isScrollOverAnchor) {
      video.classList.add('video-fixed');
    } else {
      video.classList.remove('video-fixed');
    }
  });

const mouseDown$ = Rx.Observable.fromEvent(video, 'mousedown');
const mouseUp$ = Rx.Observable.fromEvent(document, 'mouseup');
const mouseMove$ = Rx.Observable.fromEvent(document, 'mousemove');

mouseDown$
  .filter(event => video.classList.contains('video-fixed'))
  .map(event => mouseMove$.takeUntil(mouseUp$))
  .concatAll()
  .withLatestFrom(mouseDown$, (mouseMoveEvent, mouseDownEvent) => {
    return {
      x: Math.min(Math.max(mouseMoveEvent.clientX - mouseDownEvent.offsetX, 0), window.innerWidth - video.getBoundingClientRect().width),
      y: Math.min(Math.max(mouseMoveEvent.clientY - mouseDownEvent.offsetY, 0), window.innerHeight - video.getBoundingClientRect().height)
    };
  })
  .subscribe(pos => {
    video.style.top = pos.y + 'px';
    video.style.left = pos.x + 'px';
  });