It's the same application, but with 2 different interfaces. Of course, a UI toolkit that can do "write once, interact in any way" would be helpful for the implementation.

Which is essentially two applications as I said. The only difference is that they may share some state, but there are other ways to handle that too.

What I mean is that "convergence" benefit is not same UI on different devices out of pure whim, but the ability to use the same application from phone directly or using desktop monitor mouse and keyboard.

I don't mean the interfaces should actually be different. Just that "the same interface" is not the motivation by itself.

Unified interface would be good for that. If the interfaces are actually different, the app could be adopted automatically by its UI framework, or by "window manager", etc.

Just like any responsive website is.

So it's easy with what is essentially a document viewer, it's much harder when it comes to real apps, the desktop interface is dumbed down for the limitations of a phone.