> Experience has shown us that an immediate mode API is the only sane way to program GUI applications

I wonder how long till they pivot away from this belief. I feel like everyone in UI goes through this phase as some point, but in the end it doesn't scale to truly complicated UI

In my experience, it's the opposite. Immediate mode GUI, or at least a functional and declarative approach, is the only way I've seen it scale well. It's more modular and scale-independent. On the other hand, retained mode, or imperative/OOP approach to state management, becomes complicated and monstrous quickly; it's the dominant style and can be made to work OK, but typically hellish to maintain or develop beyond a certain scale.

Admittedly I'm simplifying too much and conflating paradigms. My preference is something like "functional core, imperative shell" or maybe "immediate-mode core, retained-mode shell" if that makes sense.

Yeah. Based on my personal experience I think some kind of hybrid of old-school imperative retained and declarative retained, both with granular reactivity is probably the correct balance for "serious" high-utility desktop applications. Declarative approaches are great for smaller components but become a nightmare for anything much more complex than a relatively simple mobile app while imperative requires a lot of extra legwork at the component level, and as I understand (which may be incorrect) immediate mode makes certain types of optimization more difficult.

Is Tracy complicated enough? Because it's imgui.

https://github.com/wolfpld/tracy

It's the only thing that can scale to complicated UI