I ended up choosing Dear ImGui recently https://github.com/ocornut/imgui

Depends on the complexity of the GUI you need and on other factors.

QT is OK if your GUI is very complex and you're targeting a desktop OS.

For simple GUI QT is just too big.

For GUIs without a desktop OS (mobile, embedded), QT is not a good fit either, not enough mobile support, and their embedded license can be prohibitively expensive.

I have positive experience building moderately complex commercial quality GUIs on top of this: https://github.com/memononen/nanovg

Actually QT GUI is pretty popular in embedded space(if you're not using Android that is)

in my case I want to build a file manager and after some research I ended up with Pyside2

Another way to do true cross-platform Desktop GUI will be leveraging browser on MacOS/Linux/Win, you put your core logic in GO which runs as a local http-server on your Desktop that talks to your OS underneath, and use browser/html/css/javascript to do the UI. You only need ship the GO binary, users install it and then starting to use your program via a browser.

That sounds nothing like “true cross-platform GUI” to me.

> flutter at the moment is really just mobile only, so does Kivy

I can't parse that. I know for certain that kivy runs very well on desktop platforms.

not really, it's more like running an apps on Desktop, you only have one window for example

You could also look at JavaFX. It's very modern but also has the sorts of controls you may need for complex apps.