title: Kivy ActionDropDown does not open
body: <p>In the kivy gui below, when I press the button 'Pen', an <code>ActionDropDown</code> is supposed to come. I do not see it opening. But the 'Pen' button behaves as if a <code>DropDown</code> was opened with it (e.g. you can not press on it on the first try). What is wrong?</p>
  
  <pre><code>from kivy.uix.boxlayout import BoxLayout
  from kivy.uix.actionbar import ActionDropDown
  from kivy.app import App
  from kivy.lang import Builder
  from kivy.uix.screenmanager import Screen
  
  Builder.load_string("""
  &lt;NoteScreen&gt;:
      BoxLayout:
          orientation: 'vertical'
  
          NoteScreenMenu:
  
          Label:
              text: 'i am a label'
  
  &lt;NoteScreenMenu&gt;:
      orientation: 'horizontal'
      size_hint_y: None
      height: 48
  
      ActionBar:
          pos_hint: dict(top=1)
  
          ActionView:
              use_separator: True
  
              ActionPrevious:
                  title: 'Drawing Screen'
                  with_previous: False
  
              ActionOverflow:
  
              ActionButton:
                  id: pen_button
                  text: 'Pen'
                  on_release: root.pen_drop_down.open(self)
  
              ActionButton:
                  text: 'Eraser'
  
  &lt;PenDropDown&gt;:
      Button:
          text: 'Close this'
      Button:
          text: 'btn1'
  """)
  
  
  class NoteScreen(Screen):
      pass
  
  
  class NoteScreenMenu(BoxLayout):
      def __init__(self, **kwargs):
          super(NoteScreenMenu, self).__init__(**kwargs)
          self.pen_drop_down = PenDropDown()
  
  
  class PenDropDown(ActionDropDown):
      pass
  
  
  class TheApp(App):
      def build(self):
          return NoteScreen(name='note')
  
  
  if __name__ == '__main__':
      TheApp().run()
  </code></pre>