EXWM is the natural way to do this, treating X11 windows as Emacs buffers is what it's all about. Or are you trying to get just the buffer switching part without going all the way to EXWM?
szopa[granparent] uses StumpWM, not EXWM. I am trying to get the EXWM's buffer switching behaviour in StumpWM.
By default, StumpWM lists normal buffers on C-x b, and X-windows using some other keybinding like C-t b
I'm afraid it's lost. However, note that I had never gotten it quite right, and it was for StumpWM 14 years ago.