[hjek]

> can run in background,

Syncthing runs fine in the background.

> can be launched on startup,

You can launch an app on startup regardless of what GUI toolkit it's using, or whether it's using one at all. Are you saying that you can only run a program at startup if it's using Electron?

> can have toolbar icon,

I think there's a misunderstanding here. I was talking about using the browser for the GUI of your app via an embedded HTTP-server, like Syncthing does; not replacing your entire program with a HTML-document with some JavaScript, which would clearly have some limitations.

> you as a dev have zero control over the browser used,

Well, that's also not true. You can check that the browser uses a certain user agent string, or you can add a specific version of a specific browser as a package dependency.