diff --git a/src/proto/gui_gtk_x11.pro b/src/proto/gui_gtk_x11.pro
index aaf710c..51546ae 100644
--- a/src/proto/gui_gtk_x11.pro
+++ b/src/proto/gui_gtk_x11.pro
@@ -25,6 +25,7 @@
 void gui_mch_unmaximize(void);
 void gui_mch_newfont(void);
 void gui_mch_set_shellsize(int width, int height, int min_width, int min_height, int base_width, int base_height, int direction);
+void gui_gtk_get_screen_size_of_win(GtkWidget *win, int *width, int *height);
 void gui_mch_get_screen_dimensions(int *screen_w, int *screen_h);
 void gui_mch_settitle(char_u *title, char_u *icon);
 void gui_mch_enable_menu(int showit);