added simple code to reproduce gtk3 library issue in windows/msys2 (#…

…1916)

sandbox/gtkmm/.gitignore

@@ -3,4 +3,5 @@ codebox3
 columnedit
 window_position_get_set.exe
 simpletextview.exe
+simplemenu.exe
 vte

sandbox/gtkmm/simplemenu.c

@@ -0,0 +1,50 @@
+
+// gcc -Wall simplemenu.c -o simplemenu `pkg-config --cflags --libs gtk+-3.0`
+
+// from https://zetcode.com/gui/gtk2/menusandtoolbars/
+
+#include <gtk/gtk.h>
+
+int main(int argc, char *argv[])
+{
+    GtkWidget *window;
+    GtkWidget *vbox;
+
+    GtkWidget *menubar;
+    GtkWidget *fileMenu;
+    GtkWidget *fileMi;
+    GtkWidget *nothingMi;
+    GtkWidget *label;
+
+    gtk_init(&argc, &argv);
+
+    window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
+    gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
+    gtk_window_set_default_size(GTK_WINDOW(window), 300, 200);
+    gtk_window_set_title(GTK_WINDOW(window), "Simple menu");
+
+    vbox = gtk_box_new(GTK_ORIENTATION_VERTICAL, 0);
+    gtk_container_add(GTK_CONTAINER(window), vbox);
+
+    menubar = gtk_menu_bar_new();
+    fileMenu = gtk_menu_new();
+
+    fileMi = gtk_menu_item_new_with_label("File");
+    nothingMi = gtk_menu_item_new_with_label("Do Nothing");
+
+    label = gtk_label_new("\nIf you hover for the first time the menu item\n\nFile-->Do Nothing\n\nyou see that the menu item changes colour / becomes selected.\n\nBut if you click on it, then you hover it again\nit won't become selected anymore!");
+
+    gtk_menu_item_set_submenu(GTK_MENU_ITEM(fileMi), fileMenu);
+    gtk_menu_shell_append(GTK_MENU_SHELL(fileMenu), nothingMi);
+    gtk_menu_shell_append(GTK_MENU_SHELL(menubar), fileMi);
+    gtk_box_pack_start(GTK_BOX(vbox), menubar, FALSE, FALSE, 0);
+    gtk_box_pack_start(GTK_BOX(vbox), label, FALSE, FALSE, 0);
+
+    g_signal_connect(G_OBJECT(window), "destroy", G_CALLBACK(gtk_main_quit), NULL);
+
+    gtk_widget_show_all(window);
+
+    gtk_main();
+
+    return 0;
+}