I connected my laptop to external monitor (bspwm) and the content is cut, not filling the screen width.
If i set the external monitor as the one and only, then the panel (polybar) and windows are ok. (seems that laptop screen is managing the content of the external screen)
i read wrong …
Seems like your screen or the windows are still set to your lap screen try xrandr -q to get the port name of your external monitor then try a xrandr --output <you external port> --mode 1920x1080 to check if it works and reset your windows.
This might help you.