The dev pages passed no urlOwner, so on the dev server a shell read ?tab=
but never wrote it: a clicked tab was lost on reload and a copied link
opened the first tab. The root page, every /__leaf/ page and the Studio
staging page now own the address, as the built leaves do. A framed leaf
(Studio iframes its map rows) stays unowned, so it never adds entries
to the top page's Back stack.
