Test failures, possibly related to PySide 6.6.1

Some errors I haven’t seen before:
```
======================================================================
FAIL: test_propagate_errors (traitsui.tests.editors.test_instance_editor.TestInstanceEditor)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/runner/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/traitsui/tests/editors/test_instance_editor.py", line 324, in test_propagate_errors
self.assertFalse(ok_button.inspect(IsEnabled()))
AssertionError: True is not false

======================================================================
FAIL: test_propagate_errors_switch_selection (traitsui.tests.editors.test_instance_editor.TestInstanceEditor)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/runner/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/site-packages/traitsui/tests/editors/test_instance_editor.py", line 348, in test_propagate_errors_switch_selection
self.assertFalse(ok_button.inspect(IsEnabled()))
AssertionError: True is not false

----------------------------------------------------------------------
```
On MacOS and Windows (but less reliably), Python 3.8-3.11, PySide 6.6.1

Start by running test_propagate_errors and test_propagate_errors_switch_selection in traitsui.tests.editors.test_instance_editor under the reported Python and PySide versions. Inspect why ok_button remains enabled on macOS and Windows; done means both failures pass reliably on the affected platforms.

Written by the indexing model from the issue text.