xrandr --setmonitor screenshare 2560/1x1440/1+0+0 none
geo=$(slop -f '%w/1x%h/1+%x+%y') && xrandr --setmonitor screenshare "$geo" none
xrandr --delmonitor screenshare
before this to seamlessly update the shared region — Chrome will immediately pick up the change and continue sharing the updated virtual monitor.