workaround for x86_64 ABI issue: https://github.com/ziglang/zig/issues/13598

build.zig

@@ -83,6 +83,11 @@ pub fn build(b: *std.build.Builder) !void {
 
     // Exe
     {
+        if (target.isDarwin()) {
+            // See the comment in this file
+            exe.addCSourceFile("src/renderer/metal_workaround.c", &.{});
+        }
+
         exe.setTarget(target);
         exe.setBuildMode(mode);
         exe.addOptions("build_options", exe_options);

src/renderer/Metal.zig

@@ -963,10 +963,10 @@ fn syncAtlasTexture(device: objc.Object, atlas: *const Atlas, texture: *objc.Obj
         texture.* = try initAtlasTexture(device, atlas);
     }
 
-    texture.msgSend(
+    // Workaround for: https://github.com/ziglang/zig/issues/13598
-        void,
+    ghostty_metal_replaceregion(
-        objc.sel("replaceRegion:mipmapLevel:withBytes:bytesPerRow:"),
+        texture.value,
-        .{
+        objc.sel("replaceRegion:mipmapLevel:withBytes:bytesPerRow:").value,
         MTLRegion{
             .origin = .{ .x = 0, .y = 0, .z = 0 },
             .size = .{
@@ -978,10 +978,40 @@ fn syncAtlasTexture(device: objc.Object, atlas: *const Atlas, texture: *objc.Obj
         @as(c_ulong, 0),
         atlas.data.ptr,
         @as(c_ulong, atlas.format.depth() * atlas.size),
-        },
     );
+
+    // Once the above linked issue is fixed, this is what we actually
+    // want to do:
+    //
+    // texture.msgSend(
+    //     void,
+    //     objc.sel("replaceRegion:mipmapLevel:withBytes:bytesPerRow:"),
+    //     .{
+    //         MTLRegion{
+    //             .origin = .{ .x = 0, .y = 0, .z = 0 },
+    //             .size = .{
+    //                 .width = @intCast(c_ulong, atlas.size),
+    //                 .height = @intCast(c_ulong, atlas.size),
+    //                 .depth = 1,
+    //             },
+    //         },
+    //         @as(c_ulong, 0),
+    //         atlas.data.ptr,
+    //         @as(c_ulong, atlas.format.depth() * atlas.size),
+    //     },
+    // );
+
 }
 
+extern "c" fn ghostty_metal_replaceregion(
+    objc.c.id,
+    objc.c.SEL,
+    MTLRegion,
+    c_ulong,
+    *anyopaque,
+    c_ulong,
+) void;
+
 /// Initialize the shader library.
 fn initLibrary(device: objc.Object, data: []const u8) !objc.Object {
     const source = try macos.foundation.String.createWithBytes(

src/renderer/metal_workaround.c

@@ -0,0 +1,35 @@
+// Workaround for:
+// https://github.com/ziglang/zig/issues/13598
+
+#include <objc/message.h>
+#include <objc/runtime.h>
+
+// From Metal.h
+typedef struct Origin {
+    unsigned long x;
+    unsigned long y;
+    unsigned long z;
+} Origin;
+
+typedef struct Size {
+    unsigned long width;
+    unsigned long height;
+    unsigned long depth;
+} Size;
+
+typedef struct MTLRegion {
+    Origin origin;
+    Size size;
+} MTLRegion;
+
+void ghostty_metal_replaceregion(
+    id target,
+    SEL sel,
+    MTLRegion region,
+    unsigned long offset,
+    void *ptr,
+    unsigned long len
+) {
+    void (*replaceRegion)(id, SEL, MTLRegion, unsigned long, void *, unsigned long) = (void (*)(id, SEL, MTLRegion, unsigned long, void *, unsigned long)) objc_msgSend;
+    replaceRegion(target, sel, region, offset, ptr, len);
+}