I’m not familiar with compositors, so unfortunately I don’t have a ready answer, only more questions. What makes a WindowLayoutEngine “a state-holding object w

I’m not familiar with compositors, so unfortunately I don’t have a ready answer, only more questions. What makes a WindowLayoutEngine “a state-holding object with proper structures?” How is it different from “a data tree with proper structure,” except that the former includes methods inside the class instance while the latter relies on functions to operate on the data tree?