Adopting children of an intentional null shadow root goes wrong

[annevk]

@annevk @sorvell
One thing I was thinking today about the adoption steps:
From the above example, based on the new spec, it's clear that shadowroot should be getting effective global registry or null depending on "keep custom element registry null" and the original factors (null or is using another global registry) during adoption.
But what about x-foo? I think the intended behavior is that x-foo' s registry will remain to be null. However, based on the new spec, the element has null registry originally, so during adoption in above case, it will gain the effective global registry of document, which does not align with the expected behavior.
When we set registry for null registry element during insertion in previous spec, we assign the registry based off the inclusiveDescendant's parent's registry. Do we need to do something similar when we're setting registry for null registry element during adoption? Perhaps something like "set inclusiveDescendant's parent's (or inclusiveDescendant's tree scope's) registry if it is an effective global registry in the new document, otherwise set null"

Originally posted by @ja-y-son in #1423 (comment)

[annevk]

It's tricky as what you adopt could have a mix of null and scoped registry nodes. If you look at the root (tree scope is an implementation-specific term) it doesn't work for <a-b customelementregistry>. And if you look at the parent children of scoped registry nodes will remain null. The latter seems acceptable though as that's a novel case you have to go somewhat out of your way to construct.

So that would mean the logic we want is this:

• If inclusiveDescendant's parent is null or inclusiveDescendant's parent is a DocumentFragent but not a ShadowRoot node, then do the current logic.
• Otherwise, if inclusiveDescendant is an element, inclusiveDescendant's custom element registry is null or inclusiveDescendant's custom element registry's is scoped is false, and inclusiveDescendant's parent is non-null, then set inclusiveDescendant's custom element registry to inclusiveDescendant's parent's custom element registry's effective global custom element registry.

(This would need some further changes as "effective global custom element registry" is only defined for documents right now and "custom element registry" is not generic across elements and shadow roots, but hopefully the idea is clear.)

cc @rniwa