mousedrag.js

const move$ = fromEvent(document, 'mousemove')
const down$ = fromEvent(document, 'mousedown')
const up$ = fromEvent(document, 'mouseup')

const mergeFn = downs => { // In case you want the first position
  // If you want to deltas between start and end, you might want a map that captures downs versus moves
  return move$.pipe(takeUntil($ups));
};

const paints$ = move$.pipe(
  mergeMap(mergeFn)
);