"That is what I see."
Translation:Tio estas, kion mi vidas.
Is "kio" definitely a direct object here, thereby requiring the accusative "n" suffix? As I understand, all things equal, the verb "estas"/"be" is intransitive. In any case, I think the answer has something to do with the phrase "mi vidas"/"[that] I see." What would such a phrase be called in grammar-speak?
Yes, in this instance "kio" is accusative case. It is the relative prounoun refering to "Tio" and the direct object of "mi vidas." The word "Tio" (that) is a nominative (subject) case pronoun and subject of "estas," and is refered back to by the relative object phrase "kion mi vidas."