What figure of speech is this: "the arguing sixth floor window"
I think synecdoche is most correct answer here, although it is a surprising example. Synecdoche is not always strictly about a relationship of a part to a whole. A classic example is that an athlete is a jockstrap, or "jock". The jockstrap is not part of the athlete. Another example is where a container is used to refer to its contents, e.g. "He drank the keg". I think this example is similar to that. It would be more common to say something like "The sixth floor was arguing", but your example works the same way.