Allow CJK / wide-character glyphs to render at natural size (configurable scale floor)

[wd]

Summary

Today adjustGlyphs in src/Renderer.zig scales any glyph whose font metrics don't match default_font_info so it fits exactly inside its cell. For CJK characters on macOS this consistently shrinks them to ~84% of ASCII height, which looks visually unbalanced in Chinese / Japanese / Korean text.

face-font-rescale-alist cannot work around this, because Ghostel reads the rescaled glyph height and produces a reciprocal display height that cancels the rescale out.

Why a knob is preferable to a behavior change

I understand the current behavior is the right default — it guarantees strict grid alignment, which is one of the main reasons to use Ghostel over Vterm. The maintainer's rationale in #281 makes the design intent very clear, and I'm not asking to change it.

But for buffers that aren't running a TUI app (a plain shell, or a session that's mostly CJK prose / output), users may want their CJK to render at the font's natural size — at the cost of CJK rows being slightly taller than pure-ASCII rows. That tradeoff is worth exposing as a config.

Proposed configuration

Either of these would work; option 1 is more flexible.

1. ghostel-cjk-scale-floor (defcustom, float, default 0.0)
   Clamp the computed scale: scale = max(scale_floor, computed_scale). A value of 1.0 disables shrinking entirely; 0.0 keeps current behavior. One @max after the existing @min in adjustGlyphs.

2. ghostel-disable-glyph-adjust (defcustom, bool, default nil)
   Skip adjustGlyphs entirely when non-nil. Simpler but coarser — also disables the desirable behavior for too-large fallback Nerd-Font glyphs that Nerd Font glyphs render too small #281 fixed.

I'd vote for option 1: it preserves the alignment-correction story for fallback glyphs that are way too big, while still letting CJK render at natural size when the user opts in.

Reproduction

• Doom Emacs on macOS, ghostel module pulled with ghostel-module-auto-install 'download
• doom-font = CaskaydiaCove Nerd Font Mono 14pt
• Global CJK fontset = Sarasa Mono SC (set-fontset-font t 'han ...)
• Open ghostel, type any CJK character, M-x describe-char
• Observe: display ((min-width (2)) (height 0.8421052631578947))
• Setting (setq face-font-rescale-alist '(("Sarasa Mono SC" . 1.3))) has no visible effect — Ghostel reads the rescaled height and emits a reciprocal scale.

Workaround

The only workaround today is to fork and patch src/Renderer.zig (clamp scale to 1.0, or drop the height half of the display spec) and rebuild ghostel-module.dylib. Not great for staying current with upstream.

[emil-e]

This makes a lot of sense, I will implement your suggestion, but in a slightly different way.

[wd]

I think you might not use the CJK languages. I'd like to give you a visual example to help you understand the problem I'm having.
You can see that characters other than English letters are particularly small, and the spacing between characters is quite large.

[emil-e]

I don’t use it regularly but I have tested it so I know how it works. However, the way I read Chinese is not like a native Chinese reader (like you or my partner) so it’s good to hear from others.

Could you try out the PR that I have linked? It implements your idea but just in a slightly different way technically.