I was able to find the following method of determining if a Unicode-16 character is supported by a font. Unfortunately that doesn't work for surrogate pair Unicode characters, since WCRANGE struct ...

Full disclosure: I'm working on my libui GUI framework's text API. This wraps DirectWrite on Windows, Core Text on OS X, and Pango (which uses HarfBuzz for OpenType shaping) on other Unixes. One of...