Checkbox doesn`t work if any of subitems are disabled?

[VladislavPodobriy]

Hello, everyone. Faced a strange problem in my project.

1. cascade_to_disabled = false, so disabled items are always unchecked
2. When i click N3 (see screenshot) it becomes undetermined
3. When i click N3 again it does not change its state
   Is it supposed to work this way? I want to be able to select/unselect items even if they contain disabled children
   
   JSFiddle Example