MainWindow.cmb.ui (Código)

<?xml version='1.0' encoding='UTF-8'?>
<!-- Created with Cambalache 0.7.4 -->
<interface>
  <requires lib="gtk" version="4.0"/>
  <template class="MainWindow" parent="GtkApplicationWindow">
    <property name="default-height">384</property>
    <property name="default-width">683</property>
    <property name="title">Python e GTK 4: PyGObject Gtk.Button()</property>
    <child>
      <object class="GtkBox">
        <property name="orientation">vertical</property>
        <property name="margin-bottom">12</property>
        <property name="margin-end">12</property>
        <property name="margin-start">12</property>
        <property name="margin-top">12</property>
        <property name="valign">center</property>
        <child>
          <object class="GtkButton">
            <property name="label">Clique aqui</property>
            <signal name="clicked" handler="on_button_clicked"/>
          </object>
        </child>
      </object>
    </child>
  </template>
</interface>