[benj111]

Fonts aren't really something I've dived into.

Could someone explain at what level this is happening. What classes of software/libraries are affected?

[pcwalton]

GSUB handling is done by a library called a text shaper. Examples of libraries that do shaping include the cross-platform HarfBuzz, Windows DirectWrite, and macOS Core Graphics/Core Text.

[benj111]

Thanks

So from what I understand these would be called from the UI libraries eg GTK in Linux land. I would assume window managers also, to deal with title bars.

I would further take from this that we shouldn't expect full Unicode support in virtual terminals (not emulators) in the future.....