be as portable as possible

This is important to me, which narrows down my options quite a bit.

Electron is portable across desktop OS, but unacceptably bloated (I don't want my users to have to deal with that) and buggy (I don't want to deal with that).

wxWidgets and various similar wrapper libraries exist, but on Linux most of them wrap Gtk, which in recent years has become very opinionated in UI directions that I find intolerable.

A few new cross-platform GUI toolkits have been appearing recently, but I've found all of them suffer from poor text handling, anemic widget sets, or very out-of-place look and feel (especially keyboard navigation) relative to native applications.

That leaves Qt as my only reasonable choice, at least for now. This is mostly okay, as it does a wonderful job all around. My main complaint is that using the full power of its widgets and libraries means I'm restricted to a handful of languages: C++, Python, and maybe one or two minor ones like D. Its declarative API (Qt Quick) seems to be getting more language bindings, though, so simpler apps might be possible in other languages.

Note that the landscape is different for mobile apps. I don't have a recommendation for those.