She feels at ease in her room.
I can tell that this prepositional phrase "at ease" is a subjective complement, not knowing whether it's the force of an adverb or an adjective. I think it may have the force of an adjective because you can substitute comfortable for at ease.
"Feel" is copulative here and therefore takes an adjective.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
She feels at ease in her room.
You are right. "She" is the predicand for the PP "at ease" functioning as subjective predicative complement of "feel".