use-draggable-example.2.ts

return down$.pipe(
    mergeMap(e => {
      startPosition = startPosition || { x: e.pageX, y: e.pageY };
      return move$.pipe(
        takeUntil(up$),
        map(e => ({
          x: e.pageX - startPosition.x,
          y: e.pageY - startPosition.y
        }))
      );
    })
  );