From the screen shot you send us, I see that the problem is not only in the designer of your Visual studio, but in rendering the site in browser.
It seems that the Cascading Style Sheets are not loading properly.
First you can try troubleshooting the Web Resources
Try to register your CSS using RadStyleSheetManager
and see if it makes any difference.
Tell us if the problem persists every time the page render on browser, or after Ajax request?
Would you send us the web.config
of your site (or the whole site if you can), to check it and see where the problem is coming from?
Check the attached web application, with tab strip. We tested it here with VS2005, and we were unable to replicate the issue that you are facing. Please run this application it on your Visual Studio, and tell us if it renders properly or not.
the Telerik team
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