Horizontal ruled separator does not appear under Groups

`Item('_')` does not produce a visible separator when immediately underneath a `Group`
To replicate:
```
from traits.api import *
from traitsui.api import *

class SeparatorTest(HasTraits):
view = View(
Label('Above'),
Item('_'),
Label('Below'),
Item('_'),
Group(Label('Caverns')),
Item('_'),
Group(Label('Dungeons')),
Item('_'),
Group(Label("Extra dungeons")),
Item('_'),
Label('Foundations')
)

if __name__ == "__main__":
separator = SeparatorTest()
separator.configure_traits()
```

Run the provided SeparatorTest example with TraitsUI and compare separators before and after Group entries. Trace how Item('_') and Group are handled during view construction or rendering. Done means horizontal ruled separators are visibly rendered when immediately under each Group, without regressing the other separators.

Written by the indexing model from the issue text.