Graphical glitches with Raspberry Pi 400 + LinuxCNC 2.9.4 (arm64)

The official LinuxCNC 2.9.4 image for arm64 is installed on a Raspberry Pi 400. There are strange partial graphical glitches in browsers, such as some pages or partial horizontal section of the page not displaying properly, while the rest is fine.

This was tested with Firefox ESR, but Chromium was also installed, and for example, the settings page for Chromium never displays properly. It would appear the underlying text and buttons/links are present but not visible, except by sometimes mousing over them. 

The problem appears to be only in browsers, not other applications. Youtube in either browser is also unusable, with graphical glitches on the player controls (YT buttons like play/pause/settings are either invisible or look corrupt).

Similarly, when opening PDF documents in browsers, the control buttons along the top of the document for Zoom/Download/Save etc are often invisible, but still present if moused over.

Also, Youtube video playback is extremely laggy, and that is including after removing the isolcpu=2,3 option. The goal on the Pi is just to familiarize with LinuxCNC and set up configs, not to run machines on it, so I'm not too concerned about the latency penalty in this case.. 

Has anyone else experienced this problem? 

This is really more of a Debian/RPi isue, not a LinuxCNC issue.

A Raspberry Pi has a weak GPU, why would you use it for YouTube playback?

Use a Mac Mini, an old PC, or something else with an actual GPU (a dedicated GPU is better, but not essential; a good iGPU can be good enough).

Post a screenshot so we know what you are talking about.

To tell the truth, whilst I haven't done any long term testing, I haven't seen this issue of my RPi-400.
When I build the image I put it through the basic tasks, and if I had of seen this issue arise I wouldn't have released the image, I would have deemed it defective and not fit for purpose.
Now we are going to require some more info, what monitor (brand model) screen resolution and such. And just to be sure I would like to know if this is a clean install without any extras added. If you can remember when was the last time you did an update.

And yes the RPi400 should give reasonable playback on youtube and reading pdfs shouldn't pose an issue. When I get the time I'll recheck things but as I said before I haven't seen this issue arise during testing and so far in the 6 months it has been out in the wild no one else has mentioned this.

One thing I would suggest is look at this thread, forum.linuxcnc.org/38-general-linuxcnc-q...l-images-only#325007 and take note of the video issue link & zswap link.