Invalidate model to apply sortCompletedTodosSeparately pref
Currently to see the preference change you must recreate the view, e.g. by restarting KOrganizer or toggling tree/flat view.
Solution: updateConfig() is already called when the preference is toggled. Invalidate the ProxyModel at this point so it will re-sort in place. The null check is required due to the call to setPreferences() in the constructor. I chose to check for null rather than reorder the constructor.