Stacking works fine today.. you don't need a acyclic graph, just run the solver for enough iterations and it will converge, multiple iterations is to be expecte

Stacking works fine today.. you don't need a acyclic graph, just run the solver for enough iterations and it will converge, multiple iterations is to be expected given the number of contacts and friction constraints involved. If you track the max residual you can run until it reaches ~0. The only kinda busted aspect is that collision & solver only runs at some limited update rate, so if you start with penetration it needs to resolve that, which is kinda "fake" since real world objects don't penetrate like that, so this can add extra motion and make the stack less stable, but there are various methods to work around this, all the popular physics engine have some solution or another.