I don't have time to investigate further, but it seems at least plausible that this is a regression with PySide 6.4.0. If so, we could restrict to an older version of PySide 6, with appropriate comments linking to the Pyface issue so the the version restriction can be removed once this is fixed in either Pyface or PySide 6.
Our continuous integration is broken on the main branch. We should fix this as a matter of priority before merging any other changes to main.
The cause appears to be https://github.com/enthought/pyface/issues/1163
I don't have time to investigate further, but it seems at least plausible that this is a regression with PySide 6.4.0. If so, we could restrict to an older version of PySide 6, with appropriate comments linking to the Pyface issue so the the version restriction can be removed once this is fixed in either Pyface or PySide 6.