It seems there is a problem with the TabControl and it fails to render correctly when there are collapsed TabItems in it. What I would suggest you is to make them very small (1px big). Here is the code that demonstrates my suggestion:
Tel_pages.Width = Tel_pages.Height = 1;
Tel_Layers.Width = Tel_Layers.Height = 1;
Blocks_Panel.Width = Blocks_Panel.Height = 1;
Layouts_Panel.Width = Layouts_Panel.Height = 1;
Hope this helps.
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!