Ligature support

[musjj]

It would be nice if ligature can be supported. I'm using JetBrainsMono and ligatures work just fine on GitHub code blocks. But it's not working inside firenvim.

[glacambre]

I'm not sure if this is possible to do in a generic and efficient way for Firenvim. Firenvim has no actual knowledge of the font being used and I don't know of any canvas APIs that would check if a pair of characters would be rendered with ligatures. The only solution I can think of is to render each pair of characters first "normally" and then twice by changing the second character of the pair and comparing whether the results are pixel-for-pixel identical.
It also asks some interesting questions about what neovim highlights should be used to render ligatures when the underlying cells have different highlight ids.

[alerque]

I don't know about the Canvas interface issues. Just a couple random thoughts even though I don't feel any need for this personally...

• You'll probably need to bundle and call out to an actual shaping engine to get this right: for example Harfbuzz can now be compiled to WASM.
• As for syntax highlighting issues, I'm guessing you never want to ligature across a highlight group boundary.
• There are other NeoVIM interfaces that support ligatures, for example Neovide. It is probably worth reviewing their implementations.