gtk: add config option to control GSK_RENDERER env var

src/apprt/gtk/App.zig

@@ -209,9 +209,16 @@ pub fn init(core_app: *CoreApp, opts: Options) !App {
     }
 
     if (version.runtimeAtLeast(4, 14, 0)) {
-        // We need to export GSK_RENDERER to opengl because GTK uses ngl by
-        // default after 4.14
-        _ = internal_os.setenv("GSK_RENDERER", "opengl");
+        switch (config.@"gtk-gsk-renderer") {
+            .default => {},
+            else => |renderer| {
+                // Force the GSK renderer to a specific value. After GTK 4.14 the
+                // `ngl` renderer is used by default which causes artifacts when
+                // used with Ghostty so it should be avoided.
+                log.warn("setting GSK_RENDERER={s}", .{@tagName(renderer)});
+                _ = internal_os.setenv("GSK_RENDERER", @tagName(renderer));
+            },
+        }
     }
 
     c.gtk_init();

src/config/Config.zig

@@ -1984,6 +1984,14 @@ keybind: Keybinds = .{},
 /// - `ReleaseFast`: `false`
 @"gtk-opengl-debug": bool = build_config.slow_runtime_safety,
 
+/// After GTK 4.14.0, we need to force the GSK renderer to OpenGL as the default
+/// GSK renderer is broken on some systems. If you would like to override
+/// that bekavior, set `gtk-gsk-renderer=default` and either use your system's
+/// default GSK renderer, or set the GSK_RENDERER environment variable to your
+/// renderer of choice before launching Ghostty. This setting has no effect when
+/// using versions of GTK earlier than 4.14.0.
+@"gtk-gsk-renderer": GtkGskRenderer = .opengl,
+
 /// If `true`, the Ghostty GTK application will run in single-instance mode:
 /// each new `ghostty` process launched will result in a new window if there is
 /// already a running process.
@@ -6167,6 +6175,12 @@ pub const WindowPadding = struct {
     }
 };
 
+/// See the `gtk-gsk-renderer` config.
+pub const GtkGskRenderer = enum {
+    default,
+    opengl,
+};
+
 test "parse duration" {
     inline for (Duration.units) |unit| {
         var buf: [16]u8 = undefined;