At the moment, it just overwrites any existing SIGWINCH handler.

I don't know how to do anything better -- one could try to store an existing SIGWINCH handler and call it, but it still wouldn't really work for an application that wanted to add and remove handlers.  I think such applications are rare, though -- readline mucks with the terminal in various ways, so it's unlikely that other applications will also muck with the terminal and expect to be compatible with it.