SGR 10..20 (font selection) support

[thecaralice]

ECMA-48 defines SGR 10..20 for switching between different fonts. Although I'm not aware of any terminals actually supporting this, it would be a nice feature to have.

[mitchellh]

I'm aware of this SGR from the standard but my understanding is that almost no terminals support it because it isn't clear how grid metrics are supposed to work with changing fonts.

By "grid metrics", as background: every font defines a unique grid dimension, decoration position (underline, strikethrough), etc. This is unique to the font. If you configure Ghostty (or any other terminal) with one font and then a different one, your grid size (columns x rows) may change, or underline may move higher relative to the cell baseline, etc.

My belief is SGR 10-20 was designed in an era more rampant with bitmap fonts (as opposed to scalable fonts) which fit into a predefined grid size rather than the grid size adapting to the font. This way, switching fonts is a complete non-issue. But with modern, scalable fonts, I'm not sure how this works.

I think I have some questions:

• What terminals do support this?
• For terminals that support this, how do they handle grid metrics for differing fonts?
• What programs support this and for what use case?

[thecaralice]

Isn't that problem solved already by the codepoint-map handler?

[mitchellh]

Hm, good point, but the expected use case for that is single glyphs moreso than larger runs of text. When using codepoint map (or codepoint fallback in general), we reuse the metrics from the primary font. I wonder if this would look poorly in a larger setting. It is worth experimenting with!

[thecaralice]

• What terminals do support this?

None that I used myself; pangoterm and mintty seem to support that. See also microsoft/terminal#6779

• What programs support this and for what use case?

I don't think there are a lot of programs that support it right now, with that feature being barely supported anywhere. Still, there are some possible use cases:

• Monaspace has "mix fonts together" in their philosophy, although that is not actually supported by major editors (see Mix & Match? 😢  githubnext/monaspace#30)
• artificial scripts like Tengwar, Standard Galactic Alphabet, pIqaD and Phyrexian can not be represented using codepoint-map functionality, because most of them use Basic Latin block for encoding as they don't have any allocated Unicode codepoints
• languages such as Uiua require special fonts for comfortable usage (this is what I had in mind when I thought I need an SGR 10..20 implementation)