-#if defined (USE_X_TOOLKIT) || defined (USE_GTK)
+#if defined (USE_X_TOOLKIT) && !defined (USE_GTK)
OK, so I applied this patch and ran `emacs -Q`, then did C-x 5 2 and got the usual small window and error:
(emacs:3159980): Gtk-CRITICAL **: 13:05:49.056: gtk_distribute_natural_allocation: assertion 'extra_space >= 0' failed
Now set
(setq default-frame-alist '((menu-bar-lines . 0)))
and do C-x 5 2. How does that behave?
That opens a small window with no error message.