The problem here is that there is no obvious process <-> window relation. Some windows have a PID property in their X11 Atoms list, but that's optional. After a

The problem here is that there is no obvious process <-> window relation. Some windows have a PID property in their X11 Atoms list, but that's optional. After all, X11 clients can run remotely. And then there's the problem of clients started only by childs of the process that you want to run here. As the other commenters have pointed out, the simplest and most robust thing might be to just resize all new windows to max and map them. Now you just have to make sure that only one window will be opened...