Pass the last remaining 2019 recursive ref test.

Something still seems slightly suspicious here, so it may pass all
the tests but still be wrong :/ -- but it seems at least more correct
that entering subresources (something done by implementations as they
'manually' process subschemas) should not affect dynamic_scope(), so
that piece seems correcter.

Whether we have precisely the right logic on when to append to the
dynamic scope... we shall see.

I think this may have a bit to do with picking a better value for
"unidentified" root schemas (those without $id) than we currently have
(where we use "").

referencing/_core.py

@@ -478,7 +478,7 @@ def in_subresource(self, subresource: Resource[D]) -> Resolver[D]:
         id = subresource.id()
         if id is None:
             return self
-        return self._evolve(base_uri=urljoin(self._base_uri, id))
+        return evolve(self, base_uri=urljoin(self._base_uri, id))
 
     def dynamic_scope(self) -> Iterable[tuple[URI, Registry[D]]]:
         """
@@ -492,7 +492,7 @@ def _evolve(self, base_uri: str, **kwargs: Any):
         Evolve, appending to the dynamic scope.
         """
         previous = self._previous
-        if self._base_uri and base_uri != self._base_uri:
+        if self._base_uri and (not previous or base_uri != self._base_uri):
             previous = previous.cons(self._base_uri)
         return evolve(self, base_uri=base_uri, previous=previous, **kwargs)
 

referencing/tests/test_core.py

@@ -739,7 +739,7 @@ def test_in_pointer_subresource(self):
         assert third.contents == {"ID": "grandchild"}
 
     def test_dynamic_scope(self):
-        root = ID_AND_CHILDREN.create_resource(
+        one = ID_AND_CHILDREN.create_resource(
             {
                 "ID": "http://example.com/",
                 "children": [
@@ -750,19 +750,29 @@ def test_dynamic_scope(self):
                 ],
             },
         )
-        registry = Registry().with_resource(root.id(), root)
+        two = ID_AND_CHILDREN.create_resource(
+            {
+                "ID": "http://example.com/two",
+                "children": [{"ID": "two-child/"}],
+            },
+        )
+        registry = Registry().with_resources(
+            [(one.id(), one), (two.id(), two)],
+        )
 
         resolver = registry.resolver()
         first = resolver.lookup("http://example.com/")
         second = first.resolver.lookup("#/children/0")
         third = second.resolver.lookup("grandchild")
+        fourth = third.resolver.lookup("http://example.com/two")
+        assert list(fourth.resolver.dynamic_scope()) == [
+            ("http://example.com/child/grandchild", fourth.resolver._registry),
+            ("http://example.com/child/", fourth.resolver._registry),
+        ]
         assert list(third.resolver.dynamic_scope()) == [
             ("http://example.com/child/", third.resolver._registry),
-            ("http://example.com/", third.resolver._registry),
-        ]
-        assert list(second.resolver.dynamic_scope()) == [
-            ("http://example.com/", second.resolver._registry),
         ]
+        assert list(second.resolver.dynamic_scope()) == []
         assert list(first.resolver.dynamic_scope()) == []