Also curious about this: what creates a "boundary" that prevents an invalid subtree from invalidating the parent? Does it need to be hidden, or simply placed ab

Also curious about this: what creates a "boundary" that prevents an invalid subtree from invalidating the parent? Does it need to be hidden, or simply placed absolutely etc.? The article is so well-researched otherwise that I'm sure the OP knows the answer to this, but it frustratingly didn't make it into the article itself!