Variable fonts have thin weights in v0.34.0

[Scrumplex]

Describe the bug
After updating to v0.34.0 the font weight of my terminal changed.

I bisected the git history between v0.34.0 and v0.33.1 and found d2c21ee to be the culprit. If I revert that commit on top of v0.34.0 it returns to the previous rendering.

To Reproduce
Steps to reproduce the behavior:

1. Use font_family Fira Code (https://github.com/tonsky/FiraCode/releases/tag/6.2)
2. Observe font rendering

Screenshots
My config:
Left is v0.34.0, right is v0.34.0 with the revert

NONE config:
Left is v0.34.0, right is v0.34.0 with the revert

Environment details

kitty 0.34.0 created by Kovid Goyal
Linux andromeda 6.8.4-zen1 #1-NixOS ZEN SMP PREEMPT_DYNAMIC Tue Jan  1 00:00:00 UTC 1980 x86_64

<<< Welcome to NixOS 24.05.20240416.5672bc9 (x86_64) - /dev/tty >>>

Run 'nixos-help' for the NixOS manual.

DISTRIB_CODENAME=uakari
DISTRIB_DESCRIPTION="NixOS 24.05 (Uakari)"
DISTRIB_ID=nixos
DISTRIB_RELEASE="24.05"
LSB_VERSION="24.05 (Uakari)"
Running under: Wayland (sway version 1.10-dev) missing: blur
Frozen: False
Paths:
  kitty: /nix/store/ms4k2nl5xwfl5964rxpvczsyn09nslym-kitty-0.34.0/bin/kitty
  base dir: /nix/store/ms4k2nl5xwfl5964rxpvczsyn09nslym-kitty-0.34.0/lib/kitty
  extensions dir: /nix/store/ms4k2nl5xwfl5964rxpvczsyn09nslym-kitty-0.34.0/lib/kitty/kitty
  system shell: /run/current-system/sw/bin/fish
Loaded config files:
  /home/scrumplex/.config/kitty/kitty.conf

Config options different from defaults:
background_opacity      0.975
disable_ligatures       1
font_family             Fira Code
placement_strategy      top-left
shell_integration       frozenset({'no-rc', 'no-cursor'})
symbol_map:
	U+23fb - U+23fe → Symbols Nerd Font
	U+2500 - U+259f → Symbols Nerd Font
	U+2665 - U+2665 → Symbols Nerd Font
	U+26a1 - U+26a1 → Symbols Nerd Font
	U+e000 - U+e00a → Symbols Nerd Font
	U+e0a0 - U+e0a2 → Symbols Nerd Font
	U+e0a3 - U+e0a3 → Symbols Nerd Font
	U+e0b0 - U+e0b3 → Symbols Nerd Font
	U+e0b4 - U+e0c8 → Symbols Nerd Font
	U+e0ca - U+e0ca → Symbols Nerd Font
	U+e0cc - U+e0d4 → Symbols Nerd Font
	U+e200 - U+e2a9 → Symbols Nerd Font
	U+e300 - U+e3e3 → Symbols Nerd Font
	U+e5fa - U+e62c → Symbols Nerd Font
	U+e700 - U+e7c5 → Symbols Nerd Font
	U+ea60 - U+ebeb → Symbols Nerd Font
	U+eb58 - U+eb58 → Symbols Nerd Font
	U+f000 - U+f2e0 → Symbols Nerd Font
	U+f300 - U+f32f → Symbols Nerd Font
	U+f400 - U+f532 → Symbols Nerd Font
	U+e276c - U+e2771 → Symbols Nerd Font
	U+f0001 - U+f1af0 → Symbols Nerd Font
tab_bar_style           powerline
Colors:
	active_border_color     #b4befe   
	active_tab_background   #cba6f7   
	active_tab_foreground   #11111b   
	background              #1e1e2e   
	bell_border_color       #f9e2af   
	color0                  #45475a   
	color1                  #f38ba8   
	color10                 #a6e3a1   
	color11                 #f9e2af   
	color12                 #89b4fa   
	color13                 #f5c2e7   
	color14                 #94e2d5   
	color15                 #a6adc8   
	color2                  #a6e3a1   
	color3                  #f9e2af   
	color4                  #89b4fa   
	color5                  #f5c2e7   
	color6                  #94e2d5   
	color7                  #bac2de   
	color8                  #585b70   
	color9                  #f38ba8   
	cursor                  #f5e0dc   
	cursor_text_color       #1e1e2e   
	foreground              #cdd6f4   
	inactive_border_color   #6c7086   
	inactive_tab_background #181825   
	inactive_tab_foreground #cdd6f4   
	mark1_background        #b4befe   
	mark1_foreground        #1e1e2e   
	mark2_background        #cba6f7   
	mark2_foreground        #1e1e2e   
	mark3_background        #74c7ec   
	mark3_foreground        #1e1e2e   
	selection_background    #f5e0dc   
	selection_foreground    #1e1e2e   
	tab_bar_background      #11111b   
	url_color               #f5e0dc   

Important environment variables seen by the kitty process:
	PATH                                /nix/store/ms4k2nl5xwfl5964rxpvczsyn09nslym-kitty-0.34.0/bin:/nix/store/32hx7y3vn8l5wvpzw7jgradnxnlsb6y2-imagemagick-7.1.1-29/bin:/nix/store/2nhjsfc4pg74vqnsbjmnpi7359y6f3wi-ncurses-6.4-dev/bin:/run/wrappers/bin:/home/scrumplex/.local/share/flatpak/exports/bin:/var/lib/flatpak/exports/bin:/home/scrumplex/.nix-profile/bin:/nix/profile/bin:/home/scrumplex/.local/state/nix/profile/bin:/etc/profiles/per-user/scrumplex/bin:/nix/var/nix/profiles/default/bin:/run/current-system/sw/bin
	LANG                                en_IE.UTF-8
	EDITOR                              nvim
	SHELL                               /run/current-system/sw/bin/fish
	DISPLAY                             :0
	WAYLAND_DISPLAY                     wayland-1
	USER                                scrumplex
	XCURSOR_SIZE                        24
	XDG_CONFIG_DIRS                     /etc/xdg:/home/scrumplex/.local/share/flatpak/exports/etc/xdg:/var/lib/flatpak/exports/etc/xdg:/home/scrumplex/.nix-profile/etc/xdg:/nix/profile/etc/xdg:/home/scrumplex/.local/state/nix/profile/etc/xdg:/etc/profiles/per-user/scrumplex/etc/xdg:/nix/var/nix/profiles/default/etc/xdg:/run/current-system/sw/etc/xdg
	XDG_DATA_HOME                       /home/scrumplex/.local/share
	XDG_CONFIG_HOME                     /home/scrumplex/.config
	XDG_SEAT                            seat0
	XDG_VIDEOS_DIR                      /home/scrumplex/Videos
	XDG_SESSION_TYPE                    wayland
	XDG_PICTURES_DIR                    /home/scrumplex/Pictures
	XDG_PUBLICSHARE_DIR                 /home/scrumplex/Public
	XDG_CURRENT_DESKTOP                 sway
	XDG_DOWNLOAD_DIR                    /home/scrumplex/Downloads
	XDG_MUSIC_DIR                       /home/scrumplex/Music
	XDG_TEMPLATES_DIR                   /home/scrumplex/Templates
	XDG_CACHE_HOME                      /home/scrumplex/.cache
	XDG_SESSION_CLASS                   user
	XDG_DESKTOP_DIR                     /home/scrumplex/Desktop
	XDG_VTNR                            1
	XDG_SESSION_ID                      1
	XDG_STATE_HOME                      /home/scrumplex/.local/state
	XDG_RUNTIME_DIR                     /run/user/1000
	XDG_DOCUMENTS_DIR                   /home/scrumplex/Documents
	XDG_DATA_DIRS                       /nix/store/jh4q17qfp1lgz5cr2dj3m9fn9sm16345-sway-+087226d/share:/nix/store/glky0wjlb1jc81awd8ddajr4fay1w27w-gsettings-desktop-schemas-45.0/share/gsettings-schemas/gsettings-desktop-schemas-45.0:/nix/store/207cqakykajjglbs6mwrprijwkanhnzz-gtk+3-3.24.41/share/gsettings-schemas/gtk+3-3.24.41:/home/scrumplex/.local/share/flatpak/exports/share:/var/lib/flatpak/exports/share:/home/scrumplex/.nix-profile/share:/nix/profile/share:/home/scrumplex/.local/state/nix/profile/share:/etc/profiles/per-user/scrumplex/share:/nix/var/nix/profiles/default/share:/run/current-system/sw/share

Additional context
I was able to reproduce this with an empty kitty config

[Scrumplex]

I additionally have this local fontconfig config:

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
<alias>
  <family>Fira Code</family>
  <prefer><family>Symbols Nerd Font</family></prefer>
</alias>

<alias>
  <family>Fira Code,Fira Code Light</family>
  <prefer><family>Symbols Nerd Font</family></prefer>
</alias>

<alias>
  <family>Fira Code,Fira Code Medium</family>
  <prefer><family>Symbols Nerd Font</family></prefer>
</alias>

<alias>
  <family>Fira Code,Fira Code Retina</family>
  <prefer><family>Symbols Nerd Font</family></prefer>
</alias>

<alias>
  <family>Fira Code,Fira Code SemiBold</family>
  <prefer><family>Symbols Nerd Font</family></prefer>
</alias>

<alias>
  <family>Monocraft</family>
  <prefer><family>Symbols Nerd Font</family></prefer>
</alias>
</fontconfig>

[kovidgoyal]

The commit is correct as far as I can see, change your font family to fira code medium or fira code retina or whichever variant has the weight you like. Not sure why you are aliasing fira code to nerd font though.

[Scrumplex]

It doesn't seem like any of the Fira Code variations seem to change this behavior at all.

These are the fonts in question here:

$ fc-list | grep "Fira Code"
/nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf: Fira Code,Fira Code Light:style=SemiBold
/nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf: Fira Code,Fira Code Light:style=Light,Regular
/nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf: Fira Code,Fira Code Light:style=Bold
/nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf: Fira Code,Fira Code Light:style=Medium
/nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf: Fira Code,Fira Code Light:style=Regular
/nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf: Fira Code,Fira Code Light

I also tried setting font_family Fira Code Light:style=Medium and even Bold with no change at all.

Not sure why you are aliasing fira code to nerd font though.

I use that to support Nerd Font Symbols without using a patched font in applications that don't have a feature like Kitty's symbol_map. Aliasing it like that makes fontconfig prefer nerd-fonts symbols where applicable (where NF Symbols has a glyph for a certain codepoint) while using Fira Code for anything else. This is basically the same thing as the upstream fc config: https://github.com/ryanoasis/nerd-fonts/blob/master/10-nerd-font-symbols.conf

[kovidgoyal]

Use

font_family Fira Code Light

Then run kitty with --debug-font-fallback and post the output

Repeat with

font_family Fira Code

[Scrumplex]

$ cat fira-code.conf
font_family Fira Code

$ kitty --config fira-code.conf --debug-font-fallback
[0.050] Preloaded font faces:
[0.050] normal face: /nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf:0
[0.050] bold face: /nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf:0
[0.050] italic face: /nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf:0
[0.050] bi face: /nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf:0

$ cat fira-code-light.conf
font_family Fira Code Light

$ kitty --config fira-code-light.conf --debug-font-fallback
[0.049] Preloaded font faces:
[0.049] normal face: /nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf:0
[0.049] bold face: /nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf:0
[0.049] italic face: /nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf:0
[0.049] bi face: /nix/store/fbb5h6yk4gallrfqh8770m28dymz4pzw-fira-code-6.2/share/fonts/truetype/FiraCode-VF.ttf:0

[kovidgoyal]

OK, that's strange I will take a look when I have a minute.

[kovidgoyal]

Well, looks like fontconfig is overloading index to also contain variable
font data in the upper 16 bits, sigh. And there is no way to match
variable fonts in kitty. Someday I might add support for it, but in the
meantime I suggest just uninstalling the variable variant (the one with
VF in the filename) or masking it in your fontconfig.

See #3711