Define "idle" and/or "active process". Is an unattached session with a window with a pane with a shell waiting for commands idle? Is a pane running
tail -f /dev/null idle? Or is only a session where in each pane the main process is dead idle? (remain-on-exit on). How about a pane with the main process dead, but where (grand)children still use their controlling terminal provided by the pane?