MacOS – Weird font in system windows

Well, to answer my own question: the problem was that my GPU was overheating.

I forgot to mention that this specific machine is a hackintosh. I don't know if such thing can happen to a real Mac, but anyway: here where I live it's pretty hot and dry, so there's a lot of dust accumulation inside and around my computer. The GPU cooler stopped because of this (last time I cleaned it was back in March), so I just opened up the computer case, cleaned it and now everything seems to be back to normal.

OS X normally will not display a character in the Private Use Area unless you manually set the font in the app. I understand this is intentional, since every font could have a different glyph at the same PUA code point, and it only makes sense to display something when the user specifies the font.

I think it should also work when the right font is selected via CSS or other markup in a document being read.

Some earlier versions of OS X did not have this "feature" and did display PUA stuff. I don't know how they chose which font to use when more than one had something at the code point selected.