drag window pane

  const mouseDowns$ = DOM.select('.handler').events('mousedown');
  const mouseUps$ = DOM.select('.tricycle').events('mouseup');
  const mouseMoves$ = DOM.select('.tricycle').events('mousemove');
  const mouseLeaves$ = DOM.select('.tricycle').events('mouseleave');

  const windowSizes$ = mouseDowns$
    .flatMap(mouseDown => mouseMoves$.takeUntil(mouseUps$.merge(mouseLeaves$)))
    .map(mouseDrag => {
      const clientWidth = document.querySelector('.tricycle').clientWidth;
      const fraction = (clientWidth - mouseDrag.clientX) / clientWidth;
      return fraction;
    })
    .filter(fraction => fraction < 0.9 && fraction > 0.1)
    .map(fraction => ({
      codeWidth: `${100*(1 - fraction)}%`,
      resultWidth: `${100*fraction}%`
    }));