visible_when=False in Vgroup Tabbed still visible

Hi there,

I'm facing the following issue.

If i set 2 traits items and set one of them to be not visible, it is stil visible.

I join a CME to be more explicit : Tab #1 should be not visible.

What am I doing wrong?

Thanks in advance.

Regards

PS : Debian 10 x86_64 / Python 3.7.3, with Traits libs from git source (Traits 6.1.0 dev1414 & TraitsUI 6.2.0 dev123)

```
#! /usr/bin/env python3

from traits.api import Int, Float, HasTraits
from traitsui.api import (Group, HGroup, Item, Tabbed, VGroup, View)

class Foo(HasTraits):

a = Int
b = Float

view = View(Tabbed(VGroup(HGroup(Item('a')),
label='Tab #1',
visible_when='False'),
VGroup(HGroup(Item('b')),
label='Tab #2',
visible_when='True')))

if (__name__ == '__main__'):
foo = Foo()
foo.configure_traits()
```