Not high-level model activity, but low-level technical elements like UI transitions, morphing logic etc. Everything. It should be as generic as possible, in some utility lib (module).
Also, the key is parametrizing by time. (But there can be other dimensions, too, as extensions!)
IMGUI always brings a couple of things to mind for me, which don't seem to be discussed often:
Smalltalk/Self GUIs:
AFAIK, the idea (or at least terminology) of MVC comes from the Smalltalk80 GUI. MVC seems to be intrinsically retained-mode: the "view objects" (buttons, forms, etc.) are separate to the "model objects".
Self was inspired by Smalltalk, and its Morphic UI seems to be immediate-mode: the UI objects often are the model objects. This idea is more explicit in the "Naked Objects" approach. Squeak Smalltalk uses Morphic, although its descendents (e.g. Pharo and Newspeak) seem to have replaced it (I'm not sure of their paradigm though)
Denotative, Continuous-Time Programming (DCTP):
This was originally called "Functional Reactive Programming" ( http://conal.net/papers/frp.html ), but that term is now mainly applied to discrete, event-driven systems like Elm and React, which can make some articles/papers a bit confusing to read ;)
In this approach, anything which varies over time (e.g. user input, display widgets, application state, etc.) becomes a function taking the current time as a parameter, called a "behaviour". These behaviours can be composed together in various ways to create an application's overall behaviour, still parameterised by time. The "main loop" just samples this behaviour at an appropriate rate, e.g. a fixed frame rate, or varying based on system load, how much activity is happening, etc.
This is very much IMGUI, but has some nice advantages over the fixed ticking of a traditional main loop. For example, different components of a behaviour can be sampled at different rates, e.g. rapid sampling for smooth animations, coarse sampling for expensive calculations. Interpolation behaviours can remove jerkiness (e.g. a smooth animation based on an expensive simulation).
Not high-level model activity, but low-level technical elements like UI transitions, morphing logic etc. Everything. It should be as generic as possible, in some utility lib (module).
Also, the key is parametrizing by time. (But there can be other dimensions, too, as extensions!)
Inspiration: https://news.ycombinator.com/item?id=19747122: