GTK4 在按钮中不显示助记符下划线

问：

我想在名为 Quit 的按钮中设置助记符下划线，但尽管键盘加速通过按 ALT 和指定的助记键来工作，但不会显示此下划线。在代码中，我将use-underline设置为true。但它就是行不通。我希望 Q 字符带有下划线，以便用户可以将 Q 字符识别为同义词。我做错了什么？

这是我简化的主代码：

#include <gtk/gtk.h>
#include <glib/gstdio.h>

static void activate (GtkApplication *app, gpointer user_data) {
  GtkBuilder *builder = gtk_builder_new ();
  gtk_builder_add_from_file (builder, "builder1.ui", NULL);
  GObject *window = gtk_builder_get_object (builder, "window");
  gtk_window_set_application (GTK_WINDOW (window), app);
  gtk_widget_show (GTK_WIDGET (window));
}

int main (int argc, char *argv[]) {
#ifdef GTK_SRCDIR
  g_chdir (GTK_SRCDIR);
#endif

  GtkApplication *app = gtk_application_new ("org.gtk.example", 
G_APPLICATION_FLAGS_NONE);
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);

  int status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);

  return status;
}

这是我的构建器代码：

<?xml version="1.0" encoding="UTF-8"?>
<interface>
<object id="window" class="GtkWindow">

    <property name="title">Grid</property>
    <child>
      <object id="grid" class="GtkGrid">
        <child>
          <object id="quit" class="GtkButton">
            <property name="use-underline">true</property>
            <property name="label">_Quit</property>
            <property name="has-frame">true</property>
            <layout>
              <property name="column">1</property>
              <property name="row">2</property>
              <property name="column-span">1</property>
            </layout>
          </object>
        </child>
      </object>
    </child>
  </object>
</interface>