Kivy Scrollview的颜色更改问题

from kivy.app import App
from kivy.uix.scrollview import ScrollView
from kivy.uix.label import Label
from kivy.uix.gridlayout import GridLayout
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.uix.widget import Widget
from kivy.graphics import Color, Rectangle

class CustomScrollView(ScrollView):
    def __init__(self, **kwargs):
        super(CustomScrollView, self).__init__(**kwargs)
        
        # 设置背景颜色
        with self.canvas.before:
            Color(1, 0, 0, 1)  # 设置为红色
            self.rect = Rectangle(size=self.size, pos=self.pos)
        self.bind(size=self._update_rect, pos=self._update_rect)
        
    def _update_rect(self, instance, value):
        self.rect.pos = instance.pos
        self.rect.size = instance.size

class MyApp(App):
    def build(self):
        layout = GridLayout(cols=1, spacing=10, size_hint_y=None)
        layout.bind(minimum_height=layout.setter('height'))
        
        # 添加ScrollView
        scrollview = CustomScrollView()
        layout.add_widget(scrollview)
        
        # 添加其他组件
        for i in range(50):
            label = Label(text="Label {}".format(i), size_hint_y=None, height=40)
            layout.add_widget(label)
        
        return layout

if __name__ == '__main__':
    MyApp().run()