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()

I can verify this behaviour on Python 3.8, PySide6, MacOS.

Edit: ~~It looks like listeners are not being hooked up on the root node.~~ Nope - that was a problem with my instrumentation of the code.
Edit 2: I have run out of time to look at this today - will try to dig a bit deeper in the next few days.

This has some of the feel of #1303 and related issues, but I thought that those had been resolved.

Digging in, when asking for the expanded children, etc. of the hidden node, we are getting a swallowed exception:
Qt object <PySide6.QtWidgets.QTreeWidgetItem object at 0x121365080> for node nolonger exists.

Edit: so it looks like the QTreeWidgetItem in question is being created before the tree is shown, but being deleted at some point before things are updated. Whether or not this is an issue, the problem would seem to be in part that when there is a stale tree node we may not want to ignore it. The question is then whether the node is stale because the tree editor is being finalized (in which case it would be bad to try and re-create it), or because something else happened.

Relevant code is here:

traitsui/traitsui/qt4/tree_editor.py

Lines 645 to 649 in d944a86

    
           try: 
        
               expanded, node, object = self._get_node_data(nid) 
        
           except Exception: 
        
               # The node has already been deleted. 
        
               continue

The above code is recent code, but without the try/except we would have a segfault instead of nothing happening, I suspect.

As best I can tell at the moment, this looks like it may be either a bug in Qt or a problem with the way that we use QTreeWidgetItem objects (ie. the expectations we have on their lifetime and structure). Either way, this feels like it will require a bigger change than we can fit into the next bugfix release (either to work-around or fix).

If it will take a while it would probably be better to focus on the next release as that will used in metrology. This problem can be worked around with minimal changes to the UI by setting hide_root=False

	try:
	expanded, node, object = self._get_node_data(nid)
	except Exception:
	# The node has already been deleted.
	continue