Remove the Clock.schedule for setting focus. It's not needed and it does not solve the actual issue it was introduced for.

kivy/uix/behaviors.py

@@ -878,10 +878,8 @@ class FocusBehavior(object):
             if next:
                 self.focused = False
 
-                def _fn(dt):
                 next.focused = True
 
-                Clock.schedule_once(_fn)
             return True
         return False