what should demo look like?
What should the demo look like? I can see no discernible text in it. Just lines tightly packed together. Is this how it /should/ look?
It should kinda look like this: https://bugs.freedesktop.org/attachment.cgi?id=62080
Then it failed utterly on my system.
What platform / driver?
linux 64-bit, gcc 7.1.0 glibc 2.26. Not sure what you mean by driver, unless you mean nouveau of kernel 4.14.2
This is what it looks like for me:

Humm. That's the most corrupt I've seen the demo perform! No idea where to go from here.
Maybe I'm missing the font that you use?
I'm hoping to get this resolved before I build libreoffice
Maybe I'm missing the font that you use?
No, there's no way you should get what you are getting.
I'm hoping to get this resolved before I build libreoffice
Oh, does libreoffice use GLyphy after all? I thought that was abandoned.
I don't really know, except to say that the BLFS (beyond linux from scratch) book says that glyphy is optional for LO.
What can I do to help narrow down the cause?
If you just want to move on with building your system, just skip GLyphy. It's not a strict requirement of anything.
I understand that, but it sounds like a neat thing to have. The way my system is rendering fonts now seems a little off (they look clearer is some areas than others, probably due to where the pixels are relative to the glyphs. I'm not explaining it well.)
Anyway, you were right, and the BLFS book was wrong. LO did NOT want to include glyphy.
This is what it looks like for me:
this could be a incorrect font file? eg https://github.com/behdad/glyphy/issues/37
FWIW Here's what it looks like as of today in Mint 21.1 (Ubuntu 22.04)