Comment # 2 on bug 80678 from Harald Judt

No, it doesn't work with glamor either. Besides, I'd like to use EXA because of
xv support.

Actually, I notice one difference after executing
xrandr --fb 1920x2400 --output HDMI-0 --mode 1920x1200 --scale-from 1920x2400

The vertical panel seems to stretch/extend to the new vertical size (2400).
However, the intended scaling doesn't get applied.

xrandr --verbose before and after this command shows no differences.