I once wrote such a system in JS. For every element, up could specify which elements it needed to cover (complete with a priority rule for cycles). It worked pe

I once wrote such a system in JS. For every element, up could specify which elements it needed to cover (complete with a priority rule for cycles). It worked perfectly, but it was slow, because the number of constraints rises quickly if you rely on it too much (and it needs to add all kinds of intermediate constraints to maintain the default visual hierarchy).