No concurrency

onStopDrag.ts

  onStopDrag = (id, { dx, dy }) => {
    // we capture this outside the propose closure!
    const baseBox = this.client.currentState.boxes.find(b => b.id === id);
    this.client.propose(draft => {
      const box = draft.boxes.find(b => b.id === id);
      if (box.x !== baseBox.x || box.y !== baseBox.y)
        throw "Somebody else already dragged this box!";
      box.x += dx;
      box.y += dy;
    });
  };