Please update your Telerik ASP.NET AJAX Controls suite to lastest version. I am attaching you a sample project where I use ComboBox and as you can see on the video
I haven't ran into that issue. You have also opened a ticket about the problem - if you have any further questions I will ask you to може the discussion there. Last, but not least, I would ask you avoiding threads duplication for an issue, which allows us to provide you with better support.
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