What are your definitions of good and open source?
There's: Qt, wxWidgets, Flutter, JavaFx, ...
https://en.wikipedia.org/wiki/List_of_platform-independent_GUI_libraries
What are your definitions of good and open source?
There's: Qt, wxWidgets, Flutter, JavaFx, ...
https://en.wikipedia.org/wiki/List_of_platform-independent_GUI_libraries