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Ok the screenshots are interesting. Unfortunately I haven't had a chance to check as life has got in the way.
To tell the truth when I test the images I use an old 19" 4:3 monitor. Another thing to be aware of is that the images aren't meant to be for a daily driver, the sole purpose is to run Linuxcnc which is what is concentrated on. So yes there will be packages left out that are part of a standard Desktop install.
The kernel was built using the official RPi sources, with the only mod being enabling the RT section in the kernel config. So there is really nothing fancy in that area.
It very well be an issue related to resolution, in config.txt you should be able to adjust the memory allocated to the graphics system, something you'll have to look at the config.txt documentation, 2 things to be aware of, there is no raspi-config utility so the config.txt will need to be hand edited and the location of it is different to the Raspberry OS, the main thread for the images has this info.
If you just want to explore what Linuxcnc has to offer without running a machine or connecting to any hardware there is also the option of running the amd64 version as a live session on a PC or installing in a virtual machine such as VirtualBox. RT capabilities will be non existent but it will run the simulations fine.
Another option would be to install RaspiOS and build Linuxcnc from source, a real time kernel would not be required in your case and use at ad a Run In Place install. How to do this can be found in the docs.

Thanks for the replies. I finally got a chance to check out the suggestions on the RPi400. For testing I made a new fresh image on a new SD card, and the graphical glitches could be reproduced, but it may be just an issue in chromium browser, which is not installed by default. I did not reproduce the issue in Firefox, and though it is painfully slow to load pages, Youtube does play... eventually.

The SD cards used are 64GB Kingston Canvas Select Plus. Nothing fancy, but I doubt it is the issue, since I've used multiple, and other images (default RPi OS, Ubuntu 24.04) don't have the same issue.

For reference, here are some screenshots of the glitches in chromium. Btw, I had to install xfce4-screenshooter to be able to take screenshots, as it is not installed by default in the LinuxCNC rpi image.

The first screenshot shows a horizontal section of the web page with corrupted graphics. The second is chromium's setting page which never displays correctly. The third is a pdf document opened in chromium, with the controls (download, zoom, etc) at the top not showing. The fourth shows the graphical glitches in the Youtube player controls (buttons for play, pause, settings, etc).

They work in Firefox (very slow though, which is why I installed chromium) so perhaps this is not an issue specifically with LinuxCNC's image, though I can't tell. I don't have these issues with chromium on the other RPi OS images, but I have not tried debian 12 directly. I imagine that also precludes hardware issues. The monitor is a 22" Dell at 1680x1050, though I doubt that is too relevant.

Aside from isolcpu, were there other optimizations made which may impact graphical or general performance?