We made several changes to the TabControl selection logic. However we didn't want to introduce to many breaking changes with the new release. Thus we couldn't change the PreviewSelectionChanged event implementation to be exact as users expect to be. Currently the preview event is fired after the selection properties are synchronized. Thus the TabItem's event related to IsSelected property is raised before the preview event in case the selection is changed via that property. Also when changing the selection with IsSelected property of the item and handling the preview event the IsSelectedChanged is fired twice (once before the preview and once after the preview when the selection is reversed).
We have already logged item about that feature in our public issue tracking system. You can track its status and vote for it through following link: http://www.telerik.com/support/pits.aspx#/public/silverlight/5811
Hope this helps.
the Telerik team
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