Window 10/11 native notifications
On Windows 10/11 there are native notifications, which can be used with Python though
There are multiple packages offering support, such as https://pypi.org/project/win10toast/ or https://github.com/vardecab/win10toast-click . These implementations don't integrate well with Gajim though, since they use threading to avoid blocking the application.
Furthermore, these implementations don't offer buttons for multiple callbacks, such as "Mark as Read".
It looks like for Gajim, we would have to implement native notifications ourselves.