message: stop resize operation when cursor leaves viewport
  as mouseup can't be detected outside of viewport
diff: @@ -115,16 +115,17 @@ export class Resizer {
  };
  const body = document.body;
  - const onMouseUp = (event) => {
  + const finishResize = () => {
  if (this.classNames.resizing) {
  this.container.classList.remove(this.classNames.resizing);
  }
  distributor.finish();
  - body.removeEventListener("mouseup", onMouseUp, false);
  + body.removeEventListener("mouseup", finishResize, false);
  + document.removeEventListener("mouseleave", finishResize, false);
  body.removeEventListener("mousemove", onMouseMove, false);
  };
  - // TODO: listen for mouseout on document here as well
  - body.addEventListener("mouseup", onMouseUp, false);
  + body.addEventListener("mouseup", finishResize, false);
  + document.addEventListener("mouseleave", finishResize, false);
  body.addEventListener("mousemove", onMouseMove, false);
  }
  