I've heard people using the Godot Engine to make complex UI applications. So I've tried myself writing a simple app (it required multiple windows support which was added in Godot 4 beta), and the experience was really solid. It has its own slot/signal system like Qt but because it's much more well integrated into the language/editor. The node system in Godot is similar to the DOM, and editing layout was much easier than expected. (Compared to Unity's UI system which was built on top of a component system and is more finicky to use because of this. And no and I don't want to talk about UIElements, it's unfinished and unusable in its current state). And most important you don't need to wait for C++/MOC compiler to build, because of the scripting langauge (GDScript) changes can be tested instantaneously.

Unreal Engine might be a bit overkill since it has too many dependencies built in (you don't need a state-of-the-art 3D renderer and physics engine for UI). Godot is much more lightweight than that (about 40~50 MB from my tests, at least better than Electron), but some might still think that's too much though.

I’d love to read the code. Any chance either your project is published open source or you can cite a similar open source UI application?

The Godot IDE is itself developed in Godot, according to https://medium.com/swlh/what-makes-godot-engine-great-for-ad...

Also from Godot's docs: https://docs.godotengine.org/en/stable/getting_started/intro...