I think the tests as given are rather incomplete. The tests cover data flow and input to varying degrees of complexity but completely avoid the easy or difficulty of producing the layout in question (responsiveness, resizing, alignment, etc) in addition to performance when the layout needs to be computed or updated. Coming up with a minimal API that covers the needs of a modern UI is extremely difficult, and when you add more advanced controls (beziers, transitions/animations, etc) things get even dicier.