OK, I have overscan (or underscan) such as raspi-config can handle nicely. Except I'm on Debian, not Raspbian and running raspi-config was a disaster that made an SD card unbootable.

I have a black border of about 1/2 inch on all sides. This works, sort of:
except (1) the screen edges stay the same and (2) it reverts to something more legible in a second. Variations on the scale line for 1.2, 0.8, 1.0 made a fun set of little programs but I'm just spinning my wheels. I should probably be using the transform function of xrandr instead. Somehow screen parts change but not the outside. I can do xrandr (to query) and I might see 2304 x 1296 instead of 1920 x 1080 but there's still 1/2 inch of black around the outside.

I can't find an example that I can quite grasp. I'm playing with https://github.com/qurn/xrandr-keystone-helper but this isn't quite what it's for (I have no projector/keystone).

You should indicate up front that you are installing a desktop on a Raspberry Pie and which model!
Don't make us extract that from context!

I do not have a desktop on any RPi, I use them for utility projects and small servers.
That said, if I were installing a desktop Manjaro ARM with KDE would be my first option, you can scale the desktop and display intelligently. My second choice might be mate. While I have loved Debian for i386 and X86_64 projects of all kinds, you kinda have to resolve some issues yourself because of the intended generality and wide range of possible configurations.

I have used only Raspberry Pis for several years, I didn't even think about it. They're mostly the same as Linux on an i386. This happens to be a Pi 4, 8 GB. "Desktop" is LXDE, running Debian 12 Bookworm.

This I think is more an issue with doing xrandr transforms, which are related to xinput transforms, also related to an identity matrix https://en.wikipedia.org/wiki/Identity_matrix. It's linear algebra, I was a Calculus dropout.

It requires feeding xrandr a line like:
and understanding what the numbers mean. A cookbook approach would probably work fine ("this number neans .." and this number means that) if I could find it described clearly. What I need the identity matrix for is to let me scale up the data I have (maybe 1800x950 pixels) to fit a 1920x1080 screen.

The scale function doesn't work for this: I can play with scale but the picture on the screen always has 1/2 inch block border. I made a set of scripts like
and
but that black border doesn't go away, other things change instead. If you run xrandr its default action is to list some parameters, the current size may change from 1920 to 2500 or so but it's still surrounded by black. Trying arandr right now.

I thought I had a bright idea: I'd just take a screen shot then load it into Gimp and measure. What happened was that I made a 1920x1080 image but it's bigger than my screen (because I don't see the whole screen), I can't load it and measure to the edges. arandr doesn't have very good documentation but that gave me the screen shot idea. Having an actual measurement (not 1920x1080) of my screen size would be useful.
https://imgur.com/onRtQJz
https://imgur.com/onRtQJz

You'll probably need this or something like it https://wiki.ubuntu.com/X/InputCoordinateTransformation I'm just getting back to this, not sure how far I'll get this time. This is sort of the same thing you do when setting up an LCD screen like the Raspberry Pi one. Except I did that a few years ago and promptly forgot it.

You deal with a two-dimensional array of pixels basically, but you have the ability to translate or transform them to sideways or upside down. See an xinput man page. With the RPI LCD I was adjusting away a few inactive pixels at one side, here I want to get rid of a black border around my screen. There could be a GUI for this but it's not done often. See https://duckduckgo.com/?q=xrandr+gui&t=ftsa&ia=web

SOLVED: I just needed to disable overscan, which works about the same under Debian as under Rasbian. When I tried to use a raspi-config to do it something screwed up and I had to re-image my SD to get it running again, so I didn't do that again.

The reference I found that worked was:
https://ubuntu-mate.community/t/fixi...rry-pi/20053/3

This was under Debian 12, open the file /boot/firmware/config.txt and add the line
disable_overscan=1
on a new line. Check to make sure you aren't setting it to 0 somewhere. Config.txt can be in different places I think, seems like it used to be in /boot with an early Raspbian. You should probably only have one of them. Reboot and you'll see the difference even while you're still in text mode.

I spent quite a bit of time over a few weeks trying to find an alternative but those left and top margins wouldn't budge. I was somewhat systematically using xrandr with the verbs scale, transform and panning, none worked.