build: link to the system FontConfig by default on non-macOS systems

HACKING.md

@@ -38,6 +38,15 @@ here:
 | `zig build dist`                | Builds a source tarball                                                                                                |
 | `zig build distcheck`           | Builds and validates a source tarball                                                                                  |
 
+## FontConfig and GTK
+
+Because of the global shared state that FontConfig maintains, FontConfig must
+be linked dynamically to the same system FontConfig shared library that GTK
+uses. Ghostty's default has been changed to always link to the system FontConfig
+library. If that is overridden (by specifying `-fno-sys=fontconfig` during the
+build) Ghostty may crash when trying to locate glyphs that are not available in
+the default font.
+
 ## Extra Dependencies
 
 Building Ghostty from a Git checkout on Linux requires some additional

src/build/Config.zig

@@ -424,14 +424,26 @@ pub fn init(b: *std.Build, appVersion: []const u8) !Config {
     // show up properly in `--help`.
 
     {
+        // These should default to `true` except on macOS because linking them
+        // to Ghostty statically when GTK is dynamically linked to them can
+        // cause crashes.
+        for (&[_][]const u8{
+            "fontconfig",
+        }) |dep| {
+            _ = b.systemIntegrationOption(
+                dep,
+                .{
+                    .default = if (target.result.os.tag.isDarwin()) false else true,
+                },
+            );
+        }
         // These dependencies we want to default false if we're on macOS.
         // On macOS we don't want to use system libraries because we
         // generally want a fat binary. This can be overridden with the
         // `-fsys` flag.
         for (&[_][]const u8{
             "freetype",
             "harfbuzz",
-            "fontconfig",
             "libpng",
             "zlib",
             "oniguruma",

src/build/SharedDeps.zig

@@ -200,6 +200,13 @@ pub fn add(
             if (b.systemIntegrationOption("fontconfig", .{})) {
                 step.linkSystemLibrary2("fontconfig", dynamic_link_opts);
             } else {
+                if (self.config.app_runtime == .gtk)
+                    std.debug.print(
+                        \\WARNING: Statically linking FontConfig when using the GTK app runtime is known
+                        \\to cause crashes! It is HIGHLY recommended that Ghostty be dynamically linked
+                        \\to the system FontConfig library.
+                        \\
+                    , .{});
                 step.linkLibrary(fontconfig_dep.artifact("fontconfig"));
                 try static_libs.append(
                     b.allocator,