Let's say I have an application with a non-sortable tree view for which I am persisting the column sizes using QHeaderView::{save,restore}State. Now lets say I change the view to be sortable.

If QHeaderView::restoreState is called with non-empty data, the view will not be sortable. Accordingly, my users have a different experience depending on whether or not they have run a previous version of the application.

This doesn't make sense; sortability is almost never a dynamic, persistent property of an item view. It is almost always a static property of the application that should not be modified by QHeaderView::restoreState.