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Another question: what are native controls on Linux? Qt on KDE? Gtk on Gnome? Gtk2, Gtk3, or Gtk4? Or make your own X11 widgets?

Qt on KDE and the appropriate GTK version on Gnome, yes.

In practice, Qt looks much more native on Gnome than GTK does on KDE so you should go with Qt.

You forgot GTK5 :) . Xaw3d is the best.

I did not know about those