Famous UI = f(Model) is oversimplification that was sold in slides. Real “functional UI” frameworks implement UI = f(Model, UIState) where UIState is scroll and cursor positions, view pool for virtualization, rendering caches, etc. USState is mutable and managed by the framework and the rendering engine (e.g. React + DOM, SwiftUI + UIKit + CoreAnimation). I don’t see a problem with functional approach as in React. I do see a problem with understanding of how UIState being managed between framework, ui library and rendering engine.