The two sides disagree on the algorithm used to define the border. Croatia's algorithm gives it to Serbia and better stuff to Croatia. Serbia's algorithm gives

The two sides disagree on the algorithm used to define the border. Croatia's algorithm gives it to Serbia and better stuff to Croatia. Serbia's algorithm gives it to Croatia and better stuff to Serbia. So neither claims it. But, nor will whoever loses the fight over the algorithm give it up either. That doesn't mean either side will accept some third party adding another algorithm and set of borders into the mix.