Synthetic italic not applied when Fontconfig returns an upright fallback face

[UnsaltedScholar]

Issue Description

Potential duplicate of #3791, but making this as the previous discussion seems stale.

I am using Ghostty with FiraCode Nerd Font. FiraCode does not provide native italic faces, so I expected Ghostty to synthesize italic text. Instead, italic and bold-italic are rendered upright.

Bold began working correctly after I changed the configuration from the weight-specific "FiraCode Nerd Font Med" family to the general "FiraCode Nerd Font" family. Synthetic italic still does not work.

This may be related to how Ghostty handles approximate Fontconfig matches.

If I understand correctly, in src/font/discovery.zig, Ghostty creates a Fontconfig pattern with the requested italic slant and obtains a sorted set of matching fonts. When the requested family has no italic variant, Fontconfig may still return the closest available upright face.

The returned face is then consumed in src/font/SharedGridSet.zig. The first result is added to the collection under the style that was requested:

if (try disco_it.next()) |face| {
    _ = try c.addDeferred(self.alloc, face, .{
        .style = style,
        .fallback = false,
        .size_adjustment = .none,
    });

    continue;
}

Synthetic style completion occurs later in the same function:

try c.completeStyles(self.alloc, config.@"font-synthetic-style");

So I am assuming Ghostty requests an italic FiraCode face, Fontconfig returns an upright face as its closest match, and Ghostty adds that face to the italic slot. Because the italic slot is then considered populated, completeStyles does not synthesize the missing slant.

Would the intended behavior be for Ghostty to verify that a Fontconfig result actually satisfies the requested weight or slant before assigning it to that style slot?

Addendum:
I also think a similar issue might be possible for fonts that don't have a native bold face and Fontconfig returns an upright, non-bold approximation (with inappropriate weight), but I was unable to find an example font where this is an issue.

Expected Behavior

Because Fira Code has no native italic face and font-synthetic-style should handle both italic and bold-italic, I expected Ghostty to synthesize the missing slant.

Actual Behavior

Normal text renders correctly. Bold text uses a visibly heavier face. Italic text appears identical to upright text. Bold-italic text is bold but not slanted.

Reproduction Steps

1. Open Ghostty with FiraCode Nerd Font set
2. printf '\e[0mnormal \e[1mbold\e[22m \e[3mitalic\e[23m \e[1;3mbold+italic\e[0m\n'
3. See described issue

Ghostty Logs

info: ghostty version=1.3.2-main-+ac04fc276
info: ghostty build optimize=ReleaseFast
info: runtime=.gtk
info: font_backend=.fontconfig_freetype
info: dependency harfbuzz=11.0.0
info: dependency fontconfig=21802
info: renderer=renderer.generic.Renderer(renderer.OpenGL)
info: libxev default backend=io_uring
info(os_locale): setlocale from env result=en_US.UTF-8
info(gtk): GTK version build=4.23.3 runtime=4.23.3
info(gtk): libadwaita version build=1.9.2 runtime=1.9.2
info: reading configuration file path=/home/richardb/.config/ghostty/config.ghostty
info(cli): compatibility handler for scrollback-limit handled error, you may be using a deprecated field: error.InvalidField
info(cli): compatibility handler for scrollback-limit handled error, you may be using a deprecated field: error.InvalidField
info(config): default shell src=passwd value=/bin/fish
info(config): default working directory src=passwd value=/home/richardb
warning(gtk_ghostty_application): setting GDK_DEBUG=
warning(gtk_ghostty_application): setting GDK_DISABLE=gles-api,vulkan
warning(glib): WARNING: Adwaita: The resource style-dark.css is deprecated and shouldn't be used anymore. Use style.css with media queries instead.
warning(glib): WARNING: Adwaita: The resource style-hc.css is deprecated and shouldn't be used anymore. Use style.css with media queries instead.
warning(glib): WARNING: Adwaita: The resource style-hc-dark.css is deprecated and shouldn't be used anymore. Use style.css with media queries instead.
warning(glib): WARNING: Gsk: Failed to realize renderer 'GskVulkanRenderer' for surface 'GdkWaylandToplevel': Vulkan support disabled via GDK_DISABLE
info(opengl): loaded OpenGL 4.6
info(font_shared_grid_set): font regular: FiraCode Nerd Font Med
info(font_shared_grid_set): font bold: FiraCode Nerd Font Bold
info(font_shared_grid_set): font italic: FiraCode Nerd Font Reg
info(font_shared_grid_set): font bold_italic: FiraCode Nerd Font Bold
info(io_exec): found Ghostty resources dir: /home/richardb/.local/share/ghostty
info(io_exec): shell integration automatically injected shell=.fish
warning(gtk_ghostty_application): unimplemented action=.cell_size
info(io_exec): started subcommand path=/bin/sh pid=185849
warning(stream): unimplemented CSI action: ESC [ * {  } x

Ghostty Version

Ghostty 1.3.2-main-+ac04fc276

Version
  - version: 1.3.2-main-+ac04fc276
  - channel: tip
Build Config
  - Zig version   : 0.16.0
  - build mode    : .ReleaseFast
  - app runtime   : .gtk
  - font engine   : .fontconfig_freetype
  - renderer      : renderer.generic.Renderer(renderer.OpenGL)
  - libxev        : io_uring
  - kernel version: 7.1.5-1-cachyos
  - desktop env   : other
  - GTK version   :
    build         : 4.23.3
    runtime       : 4.23.3
  - libadwaita    : enabled
    build         : 1.9.2
    runtime       : 1.9.2
  - libX11        : enabled
  - libwayland    : enabled

OS Version Information

CachyOS Linux

(Linux only) Display Server

Wayland

(Linux only) Desktop Environment/Window Manager

KDE Plasma 6.7.3

Minimal Ghostty Configuration

font-family = "FiraCode Nerd Font"
font-style = "Medium" 
font-style-bold = default 
font-style-italic = default 
font-style-bold-italic = default 
font-synthetic-style = bold,italic,bold-italic

Additional Relevant Configuration

Built with system fontconfig (to fix log errors):
zig build -Doptimize=ReleaseFast -p ~/.local/ -fsys=fontconfig

I acknowledge that:

• I have reviewed the FAQ and confirm that my issue is NOT among them.
• I have searched the Ghostty repository (both open and closed Discussions and Issues) and confirm this is not a duplicate of an existing issue or discussion.
• I have checked the "Preview" tab on all text fields to ensure that everything looks right, and have wrapped all configuration and code in code blocks with a group of three backticks (```) on separate lines.

[UnsaltedScholar]

This is a very minimal patch against f124c42 that seems to fix this issue (for the italics case):

diff --git a/src/font/discovery.zig b/src/font/discovery.zig
--- a/src/font/discovery.zig
+++ b/src/font/discovery.zig
@@ -315,6 +315,14 @@ pub const Fontconfig = struct {

         pub fn next(self: *DiscoverIterator) fontconfig.Error!?DeferredFace {
             if (self.i >= self.fonts.len) return null;
+
+            const requested_slant: fontconfig.Slant =
+                @enumFromInt((try self.pattern.get(.slant, 0)).integer);
+            const matched_slant: fontconfig.Slant =
+                @enumFromInt((try self.fonts[self.i].get(.slant, 0)).integer);
+            if (requested_slant != .roman and matched_slant == .roman) {
+                return null;
+            }

             // Get the copied pattern from our fontset that has the
             // attributes configured for rendering.

Edit:
Here is an alternate version that should probably catch any cases for bold as well (assuming they exist). Unsure if this inclusion is really the correct way of doing things:

diff --git a/src/font/discovery.zig b/src/font/discovery.zig
--- a/src/font/discovery.zig
+++ b/src/font/discovery.zig
@@ -315,6 +315,28 @@ pub const Fontconfig = struct {

         pub fn next(self: *DiscoverIterator) fontconfig.Error!?DeferredFace {
             if (self.i >= self.fonts.len) return null;
+
+            const requested_slant: fontconfig.Slant =
+                @enumFromInt((try self.pattern.get(.slant, 0)).integer);
+            const matched_slant: fontconfig.Slant =
+                @enumFromInt((try self.fonts[self.i].get(.slant, 0)).integer);
+
+            const requested_weight =
+                (try self.pattern.get(.weight, 0)).integer;
+            const matched_weight = try self.fonts[self.i].get(.weight, 0);
+            const bold_min = @intFromEnum(fontconfig.Weight.demibold);
+            const weight_mismatch = requested_weight >= bold_min and
+                switch (matched_weight) {
+                    .integer => |weight| weight < bold_min,
+                    .range => false,
+                    else => true,
+                };
+
+            if ((requested_slant != .roman and matched_slant == .roman) or
+                weight_mismatch)
+            {
+                return null;
+            }

             // Get the copied pattern from our fontset that has the
             // attributes configured for rendering.