display – How can I stop the system compensating for xrandr’s screen width setting?

I have an HDMI monitor connected, which overscans horizontally. I can’t see the extreme left or right of the desktop.

If I correct for this with, say,

xrandr --output HDMI-1 --transform 0.5,0,-50,0,1,-10,0,0,1

the horizontal and vertical translations work fine. But the scaling works only momentarily; immediately afterwards, the desktop redraws.

For example, suppose I scale the horizontal dimension by 0.5. Now the desktop takes up only half the screen. Then it redraws so that it takes up the full width of the screen again, but the width of each pixel remains the same as I set (so that the characters are half as wide).

How do I tell it to stop doing that? I’m running a GNOME desktop on focal.