[kazinator]

"anata ga kukkii wo taberu" (you eat the cookie)

"anata ga kukkii wo taberu no wo mita yo" (I saw you eat the cookie)

"Anata" is now the subject of a subordinate clause.

I think it can be topicalized (in the sense of becoming the grammatical topic, marked by ha) like this:

"Anata ha, kukkii wo taberu no wo mita yo". (As for you, I saw (you) eat the cookie.)

The subordinate clause now has an unspecified/implicit subject, and that identifies with the topic that was introduced.

I think an extraposition of "anata" (into a non-topical position) is possible like this, where subordinate clause minus its subject is now the topic:

"kukkii wo taberu no wo mita ha, anata da yo". (the one I saw eating the cookie is you)