I think there should be a way to turn off the automatic font changing. For example, when I type certain characters (such as 「 and 」) and the font is set to Fairfax HD with a font size of 12pt, it changes to a NSimSun (at least it would've if I hadn't went into the specific setting and changed the font for the "Asian" character set to Fairfax HD) at a size of 10.5pt because 「 and 」 are part of the "Asian" character set, while the other characters I was using (characters in the UCSUR, which are assigned to private use characters) aren't. This is very annoying, so I think that there should be a way to turn this feature off.
Font fall back/substitution is controlled by the os. Additionally, if you use a font heavy with PUA, the os font fallback will be chaotic. Unicode PUA codepoints are meaningless except when explicitly applied from the font containing them--there is no effective way to control that, locale or language of the glyph or as assigned to word is not sufficient. Use a charmap utility, like BableMap on Windows, to explore the coverage of the font. LibreOffice's Special Character Dialog also works, but the collapsed character charts are a bit annoying to work with when looking at the PUA glyph coverage, or a fonts actual BMP/SMP glyphs. Something not in the font gets os controlled fallback.
(In reply to V Stuart Foote from comment #1) > Font fall back/substitution is controlled by the os. > > Additionally, if you use a font heavy with PUA, the os font fallback will be > chaotic. Unicode PUA codepoints are meaningless except when explicitly > applied from the font containing them--there is no effective way to control > that, locale or language of the glyph or as assigned to word is not > sufficient. > > Use a charmap utility, like BableMap on Windows, to explore the coverage of > the font. LibreOffice's Special Character Dialog also works, but the > collapsed character charts are a bit annoying to work with when looking at > the PUA glyph coverage, or a fonts actual BMP/SMP glyphs. Something not in > the font gets os controlled fallback. The font does support the characters I mentioned, and I used the LibreOffice settings to make it switch to Fairfax HD instead of NSimSun, not those of the OS. This also happens when I'm using no private use characters (like when I paste letters of a certain script into something set to that script's version of Noto Sans and it changes to Lucida Sans)