I love Flutter. It's super dumb that they are not pushing hard to every platform (you could render to the web and basically get the desktop for free!). I would love to use it for a desktop app, or maybe on the web.
There are other ways to get the same juice, though.
I love MOAI, for the same reasons as I love Flutter. Except MOAI has a better license .. even if the responsibilities for using it as a non-Game GUI are pretty high. You can do it, but you've gotta have Lua-foo.
In my case, I'm going to glom Ultralig.ht onto MOAI some day soon, and see if it makes the Flutter desktop problem irrelevant...