Bug: Tree editor does not update in the UI

I've attached a semi-minimal example, but a tree editor item does not update in the UI when data is added to the object if the tree editor has `hide_root=True`, the item is `style='custom'` and has a `visible_when` criteria, and the Group has `scrollable=True`.

I am not seeing any error message when running this code.

Semi-Minimal example

```
from traits.api import Instance, List, Str, Button, Bool, HasTraits
from traitsui.api import (
Group, Item, ModelView, TreeEditor, TreeNode, View, UItem,)

class Data_Group(HasTraits):
name = Str

class Holder(HasTraits):
name = Str

Data_Groups = List(Instance(Data_Group))

class SimpleExample(ModelView):
model = Instance(Holder)

# Adds another Data_Group
button = Button('Add')

def _button_fired(self):
yeah = Data_Group(name='frank')
self.model.Data_Groups.append(yeah)

def default_traits_view(self):

tree_editor = TreeEditor(
nodes=[
TreeNode(
node_for=[Holder],
auto_open=True,
children='Data_Groups',
label='',
),
TreeNode(
node_for=[Data_Group],
auto_open=False,
label='name',
),
],
# If hide_root is commented out(or set to false) it works
hide_root=True,
orientation='vertical',
)

return View(
Group(
Item(
name='model',
editor=tree_editor,
resizable=True,
show_label=False,
),
Item('button'),
show_border=True,
),
title='Measurement Groups',
width=500,
resizable=True,
)

class SimpleView(ModelView):

model = Instance(SimpleExample)

show_measurement_groups_table = Bool()

def default_traits_view(self):
return View(
Group(
Item(name='show_measurement_groups_table',
),
UItem(
'model',
# If style is commented out it works
style='custom',
# If visible_when is commented out it works
visible_when='show_measurement_groups_table',
),
),
resizable=True,
# If scrollable is commented out it works
scrollable=True,
)

if __name__ == '__main__':

temp = Holder(name='test')
collection = SimpleExample(model=temp)
mcv = SimpleView(model=collection)
mcv.configure_traits()
```