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

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.