Kivy中如何让嵌套的垂直BoxLayout保持顶部对齐？

我用Kivy开发带屏幕导航菜单的应用，菜单是包含按钮的垂直BoxLayout，用来切换屏幕，这个菜单嵌套在水平BoxLayout里，布局内还包含ScreenManager。我尝试通过pos_hint: {"top":1}让这个垂直BoxLayout置顶，但未达到预期效果。此外，打印interface.ids["menu"].height得到100，interface.ids["menu"].minimum_height得到0，这与菜单内控件总高度400不符。

Python代码 #

from kivy.lang import Builder
from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.screenmanager import ScreenManager, Screen

class Menu(BoxLayout):
    pass

class ArtistTab(Screen):
    pass

class SongTab(Screen):
    pass

class AlbumTab(Screen):
    pass

class MuDoApp(App):
    def build(self):
        interface = Builder.load_file('main.kv')
        sm = ScreenManager()
        sng_tab = SongTab(name="song")
        art_tab = ArtistTab(name="artist")
        alb_tab = AlbumTab(name="album")
        sm.add_widget(art_tab)
        sm.add_widget(alb_tab)
        sm.add_widget(sng_tab)
        interface.add_widget(sm)
        print(interface.ids["menu"].height)
        print(interface.ids["menu"].minimum_height)
        return interface
    
if __name__ == '__main__':
    app = MuDoApp()
    app.run()

KV文件内容 #

BoxLayout:
    Menu:
        orientation: "vertical"
        pos_hint: {"y":1}
        id: menu
        
        Label:
            size_hint: (None,None)
            size: 500,100
            text: "Download by ..."
        Button:
            size_hint: (None,None)
            size: 300,100
            text: "songtitel"
        Button:
            size_hint: (None,None)
            size: 300,100
            text: "artist"
        Button
            size_hint: (None,None)
            size: 300,100
            text: "album"
        
        
<ArtistTab>:
    BoxLayout:
        id: artist_tab
        TextInput:
            size_hint: (None,None)
            width: 500
        Button:
            size_hint: (None,None)
            width: 300
            text: "download"

不同pos_hint参数测试结果 #

• pos_hint: {"top":1}：菜单部分被挤出屏幕顶部
• pos_hint: {"top":0}：菜单贴在屏幕底部
• pos_hint: {"top":.7}：菜单位置在屏幕中部偏上
• pos_hint: {"y":1}：菜单完全在屏幕顶部外，不可见
• pos_hint: {"y":0}：菜单贴在屏幕底部
• pos_hint: {"y":.7}：菜单位置在屏幕中部偏上

疑问 #

是我对pos_hint的理解有误，还是该属性在BoxLayout嵌套场景下无法生效？

内容的提问来源于stack exchange，提问作者Becks