Linux is not something that encompasses a GUI, so this is a misguided way to frame the underlying concern. One might as well ask what toolkit is native on smartphones, or what language is natively spoken by humans.
We should instead ask what is native on Plasma, or on GNOME.
"Oh, bother. That would mean I have to think of four things instead of three."
Well, yes, Pooh. But the things that make them different are the things that make them, them.
"Just build native UIs. I don't know why cross platform UI has been such a hobbyhorse for so many for so long: it's a stupid idea."
The stupid idea is thinking that building native UIs is targeting a clean, non-moving target and implying it is not astronomically harder than doing something cross platform that is between mediocre and good enough for everybody.
Whenever someone says “just do native” it always turns out they have a cockamamie definition of native, otherwise they would realize the gravity and general unreasonableness of what they were asking.
On a side note, what’s a major* reasonably complex app that isn’t a music player or an IRC client, or something with a simple shell around a canvas like a browser, that targets both Plasma and GNOME natively.
* And whether anyone likes it or not, few people care (as revealed by $$, not bitching on a forum). Blame electron, the web, but even Microsoft cannot even define and maintain a native LnF for their core operating system. As I alluded to, the only small community that cares are those on MacOS and iOS, where there is a HIG that more than 2 people have read and give a shit about, and there is a small market of people that value well integrated Mac apps that will actually spend money.
Targeting Win USER32 +/- XAML, Qt, GTK, and AppKit is costly and thankless when you probably could have just used Qt, JavaFX, or Flutter, etc.
Linux is not something that encompasses a GUI, so this is a misguided way to frame the underlying concern. One might as well ask what toolkit is native on smartphones, or what language is natively spoken by humans.
We should instead ask what is native on Plasma, or on GNOME.
"Oh, bother. That would mean I have to think of four things instead of three."
Well, yes, Pooh. But the things that make them different are the things that make them, them.