I was thinking about similar thing recently. The UI state should be isomorphic to the state of the application (which is arguably hard to do with the command line, but I digress).

So every action that user can take in the UI has clear correspondence in valid change of the application state.

Also, this would affect composability. If the state of the application of composed of smaller elements, the UI should be composable too.

So just be defining the application state (this perhaps includes edited document in editors), and possible transitions (which is a category), one should get a skeleton of how UI would behave.