its being fully assembled = possessive adjective-gerund (noun)-adverb-predicate adjective (complement of the linking gerund)
it being fully assembled = pronoun-participle (adjective)-adverb-predicate adjective (complement of the linking participle)
Traditionally, the first construction has been preferred. It has not been that long that the second construction has achieved any degree of respectability at all.
By the way, while I often prefer the second type, I prefer the gerund form in this sentence.
Teachers can make mistakes. Don't be too hard on him/her (unless it becomes a habit).
Student or Learner