[gtkmm-documentation] application/menubar example: Make the keyboard shortcuts work

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 58116667c83a72c0f5b8bade56187c5e5cc98194
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Mon Mar 28 17:18:07 2022 +0200

    application/menubar example: Make the keyboard shortcuts work
    
    and avoid the Gtk-CRITICAL message:
    gtk_widget_child_focus: assertion 'GTK_IS_WIDGET (widget)' failed
    by adding a child widget to the ExampleWindow.

 examples/book/application/menubar/exampleapplication.cc | 17 ++++++++---------
 examples/book/application/menubar/examplewindow.cc      |  5 +++++
 examples/book/application/menubar/examplewindow.h       |  2 ++
 3 files changed, 15 insertions(+), 9 deletions(-)
---
diff --git a/examples/book/application/menubar/exampleapplication.cc 
b/examples/book/application/menubar/exampleapplication.cc
index 10a82dc..612295a 100644
--- a/examples/book/application/menubar/exampleapplication.cc
+++ b/examples/book/application/menubar/exampleapplication.cc
@@ -52,14 +52,8 @@ void ExampleApplication::on_startup()
 
   auto submenu_edit = Gio::Menu::create();
   submenu_edit->append("_Cut", "win.cut");
-  auto item = Gio::MenuItem::create("_Copy", "win.copy");
-  //Setting "accel" works, but might be deprecated soon: See 
https://bugzilla.gnome.org/show_bug.cgi?id=708908
-  //When it is deprecated, use Gtk::Application::set_accel_for_action() instead.
-  item->set_attribute_value("accel", Glib::Variant<Glib::ustring>::create("<Primary>c"));
-  submenu_edit->append_item(item);
-  item = Gio::MenuItem::create("_Paste", "win.paste");
-  item->set_attribute_value("accel", Glib::Variant<Glib::ustring>::create("<Primary>v"));
-  submenu_edit->append_item(item);
+  submenu_edit->append("_Copy", "win.copy");
+  submenu_edit->append("_Paste", "win.paste");
   win_menu->append_submenu("Edit", submenu_edit);
 
   auto submenu_notification = Gio::Menu::create();
@@ -69,6 +63,12 @@ void ExampleApplication::on_startup()
 
   set_menubar(win_menu);
 
+  // Set accelerator keys:
+  set_accel_for_action("app.quit", "<Primary>q");
+  set_accel_for_action("win.cut", "<Primary>x");
+  set_accel_for_action("win.copy", "<Primary>c");
+  set_accel_for_action("win.paste", "<Primary>v");
+
   //Create an action with a parameter. This action can be activated from
   //a Gio::Notification, sent by the Notification/win.send-note menu selection.
   auto action_print =
@@ -81,7 +81,6 @@ void ExampleApplication::on_startup()
 void ExampleApplication::create_window()
 {
   auto window = new ExampleWindow();
-  window->set_default_size(300, 100);
 
   //Make sure that the application runs for as long this window is still open:
   add_window(*window);
diff --git a/examples/book/application/menubar/examplewindow.cc 
b/examples/book/application/menubar/examplewindow.cc
index eadc758..9f80fa7 100644
--- a/examples/book/application/menubar/examplewindow.cc
+++ b/examples/book/application/menubar/examplewindow.cc
@@ -21,6 +21,11 @@
 ExampleWindow::ExampleWindow()
 {
   set_title("Gtk::Application example");
+  set_default_size(350, 100);
+
+  // Some GTK functions can misbehave if a window does not have a child.
+  // An empty Box is acceptable.
+  set_child(m_box);
 
   // We can use add_action because Gtk::ApplicationWindow derives from ActionMap.
   // This Action Map uses a "win." prefix for the actions.
diff --git a/examples/book/application/menubar/examplewindow.h 
b/examples/book/application/menubar/examplewindow.h
index a982ce1..f7142a2 100644
--- a/examples/book/application/menubar/examplewindow.h
+++ b/examples/book/application/menubar/examplewindow.h
@@ -29,6 +29,8 @@ private:
   void on_action_close();
   void on_action_send_notification();
   void on_action_withdraw_notification();
+
+  Gtk::Box m_box;
 };
 
 #endif /* GTKMM_EXAMPLEWINDOW_H */