macOS: reduce mechanism to create windows back to one

src/Surface.zig

@@ -2119,15 +2119,11 @@ pub fn performBindingAction(self: *Surface, action: input.Binding.Action) !void
         } else log.warn("dev mode was not compiled into this binary", .{}),
 
         .new_window => {
-            if (@hasDecl(apprt.Surface, "newWindow")) {
-                try self.rt_surface.newWindow();
-            } else {
             _ = self.app_mailbox.push(.{
                 .new_window = .{
                     .parent = self,
                 },
             }, .{ .instant = {} });
-            }
         },
 
         .new_tab => {

src/apprt/embedded.zig

@@ -151,6 +151,17 @@ pub const App = struct {
         _ = surface;
         // No-op, we use a threaded interface so we're constantly drawing.
     }
+
+    pub fn newWindow(self: *App, parent: ?*CoreSurface) !void {
+        _ = self;
+
+        // Right now we only support creating a new window with a parent
+        // through this code.
+        // The other case is handled by the embedding runtime.
+        if (parent) |surface| {
+            try surface.rt_surface.newWindow();
+        }
+    }
 };
 
 pub const Surface = struct {