The idea of a copular was imported from Latin, and despite the best efforts of prescriptivists has not I believe ever described English as she is spoke.
In particular, I regard the accusative as appropriate and the nominative as inappropriate - viz. the verb to be is just another verb and does not have any special status in English other than it's role as an auxiliary (which extends its predicative role to participles in their alter ego role as adjectives). So I prefer the first rather than the second of...
+ It was me
* It was I
It is/was deciding/decided
(and similar the auxiliary functions of were/has in the verb slot derive from their main verb semantics)
For your example
It is/was clean
It is/was clean off its hinges
It is/was off its hinges
It is/was flat
It is/was flat on the floor
It is/was on the floor
these are all copular constructions in the sense of involving the verb to be.
It blew/flew/swung/fell [clean|awkardly] [off its hinges] [onto the floor] [with a crash]
It fell/landed [flat|dead] [on the floor] [with a crash]
represents the usual verbs that can be intransitive (the wind can blow, the door can swing, the bird can land, the cup can fall), but can also be biintranstive, meaning can take an (one indirect object (or more) indicating where it got to in a literal physical sense or in a metaphorical resultative sense or as a state or manner (flat, clean, dead, awkwardly). To the extent that these adverbial/prepositional phrases are optional they can also be regarded as margins of the clause, but in fact there is a strong desire to complete these verbs with at least one object, although that could be delayed for effect to the next clause/sentence, optionally using recapitulation
It swung... [fell] clean off its hinges... [crashing] flat onto the floor...
In my view it doesn't have anything to do with copulars, which I don't believe exist in English anyway in the sense of a grammatically distinct class, as opposed to a semantically contrived class (is/seems/...) or a misguidedly imported class.