The one-key registry makes nested installation worth a regression test too. I'd install two custom container types at the window root, override just one inside a child view, and assert that the other still resolves from the parent rather than C.shared. Then remove the child and check the parent's instance again. Distinct marker services make a lost registry entry visible; otherwise the compatibility fallback could let an accidental replacement of the whole dictionary look correct. That would complement the implicit-key-path test with a check of environment inheritance.