Sorry Luckyman, I don't understand the issue still exists in 4.0 final. Do you mean you just discovered the bug but do not need a solution as you are not trying to use it in this way?
Yes, I'm just discovered the bug.(I'm try to help to ExtJs team to improve quality.)
I'm not using ExtJs 4 in my projects (may be in future).
Now I want to migrate to ExtJs 4, but a lot of issues and troubles with migration stop me.
I haven't time to find the solution.