Qt would have been my first choice for a long time. Unfortunately the Qt Company has changed their pricing model a few years ago and charges way too much these days. (3950$ per developer and year for desktop and mobile applications. For embedded systems they don't even list public prices...). That said even the open source version is less interesting since one doesn't get official long term security and bug fix updates without continuously migrating to the newest releases.

I settled for Yue: https://github.com/yue/yue It's been around for several years, though definitely doesn't have the developer or user base of other well-known frameworks. The deciding factor for me was that is has well maintained Lua bindings as part of the core project, alongside JavaScript (Node.js) and C++.

I didn't have much luck with libui (crashes, missing features, etc), and various immediate mode alternatives just require too many dependencies and other work that made integration painful. Plus, Lua bindings for these were always stale. In fact, Lua binding quality is pretty poor all around, including GTK, Qt, WxWidgets, and FLTK.