C++ was the GUI king during the 1990's, and none of the Rust toolkits is half as good as the surviving frameworks, like C++ Builder, Qt, wxWidgets, heck even MFC has better tooling.
Trying to defer to native widget rendering per platform was a mistake, and every time I've touched wxWidgets in the past decade and a half I've regretted it.
FLTK on the other hand is ugly as sin, but I've found it reliable enough to not act in surprising ways, and it's also small enough to where you can vendor it and build it alongside your program.
