FLTK
Fast Light Toolkit
https://www.fltk.org/
ウィジェットツールキット
（
Widget toolkit
）
GTK
gtkmm
(C++ binding of GTK)
FOX toolkit
IUP
(software) - a multi-platform toolkit to build native graphical user interfaces
Juce
Qt
(software)
wxWidgets
- cross platform open source C++ widgets toolkit developed by community
U++
List of widget toolkits
https://ja.wikipedia.org/wiki/FLTK
https://en.wikipedia.org/wiki/FLTK