var dragTarget = document.getElementById('drag');

​

var mouseup   = Rx.Observable.fromEvent(document,   'mouseup');

var mousemove = Rx.Observable.fromEvent(document,   'mousemove');

var mousedown = Rx.Observable.fromEvent(dragTarget, 'mousedown');

​

function prioritize(s$) {

  var first = new Rx.Subject();

  var second = s$.do(x => first.onNext(x)).share();

  return [

    Rx.Observable.using(

      () => second.subscribe(() => {}),

      () => first

    ),

    second

  ];

}

​

var mousedowns = prioritize(mousedown);

​

var dragstart = mousedowns[0].flatMap(() =>

 mousemove

   .where(x => x.movementX !== 0 || x.movementY !== 0)

   .takeUntil(mouseup)

   .take(1)

);

​

var dragmove = mousedowns[1].flatMap(() =>

  mousemove

    .where(x => x.movementX !== 0 || x.movementY !== 0)

    .takeUntil(mouseup)

);

​

dragmove.subscribe(() => console.log('dragmove'));

dragstart.subscribe(() => console.log('dragstart'));

​

​