At a hacker conference in the early 2000s, I saw a maybe 5" cash register CRT screen on a tower server case. That was cool.

It inspired me much later to buy a 7" LCD for the same purpose. You can find them as Raspberry Pi accessories. Some of them have HDMI input, most use USB for power, and they are cheap - about 50€. The downside is that they tend to be almost bare circuit boards with a bit of plexiglass framing + stand.

There are also "DVD watching screens" for car headrests, which are more sturdy with a thick case. The downside there is that power supply (12, 1A or so) is more of a hassle, and good luck finding one without overscan. It's not in the specs if they have it or if it can be disabled.

Oh this reminds me, in 2008 I built a PC case where the (normally clear acrylic) side of the computer was a backlit LCD monitor, and it could remain working and pivot outward to access inside the PC.

It would be nice if motherboards can POST to a mini display like this even without any igpu in the processor.