I logged this as a bug report. You can monitor it via this feedback portal item—http://feedback.telerik.com/Project/108/Feedback/Details/187096
. You can also try whether the workaround suggested makes things better.
Also, if the Toggle Fullscreen button is hidden for some reason you can use F11 to restore the size.
Do you need help with upgrading your ASP.NET AJAX, WPF or WinForms projects? Check the Telerik API Analyzer
and share your thoughts.