For macOS: XCode + SwiftUI/Cocoa + mdz_containers/mdz_editor libs looks promising.
For Linux: GTK + mdz_containers/mdz_editor libs seems to work. Code compiles and works just fine under Ubuntu 18.04 (showing main window, menu-bar, tabs-view, text-files under separate tabs) using GTK 2.0.
It works in both GNOME:
Weird is: under Ubuntu 14.04 the same code compiles and runs but shows only empty main window.
I decided not to touch Qt yet, because of commercial license limitations.
Also option of XLib + GLX looks interesting. Kind of “maximal portability” solution but without “native” look and fill.