Use stack layout

main.py

@@ -6,6 +6,7 @@ kivy.require('2.2.1')
 
 from kivy.app import App
 from kivy.uix.boxlayout import BoxLayout
+from kivy.uix.stacklayout import StackLayout
 from kivy.uix.button import Button
 from kivy.uix.label import Label
 from kivy.uix.popup import Popup
@@ -31,7 +32,7 @@ class SingleBridgeScreen(Screen):
         self.sm = sm
         self.tinge = tinge
         self.bridge = bridge
-        self.layout = BoxLayout()
+        self.layout = StackLayout()
         self.add_widget(self.layout)
         self.set_group_buttons()