Gtk

Qt

The list is actually quite extensive:

https://en.wikipedia.org/wiki/List_of_platform-independent_G...

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For mobile, clean separations also helps. It is definitely possible - and less complex than you'd expect - to have core functionality in a shared library, wired up to platform-specific native GUI toolkits.

But your question illustrates the problem. The pervasive presence of toolkits that add layer upon layer to create "cross platform" are the new norm. People are literally losing awareness that other options exist - much to the detriment of end users.

We have built an entire industry around such tooling - and long ago stopped questioning what value it brings.