It's those multi-client scenarios for which the API-first approach is a perfect fit, actually. Once the API is hammered out, it forms the common foundation for building gui's, cli's and even machine-to-machine interfaces. As a stepping stone, web applications can be deployed as hybrid mobile apps using something like Apache Cordova, even as fully native apps for Android and iOS are under development. Once you've implemented something like this, it's hard to go back to monolithic codebases. Caveat: this is predicated on having an API server and requiring an Internet connection. Writing a connectionless app this way would require some additional effort, though still quite do-able.

I don't know if such a thing exists yet, but how would Java compare to a shared Electron?

[ Found the issue. https://github.com/electron/electron/issues/673 ]