Yes, the video has been recorder with our latest official release - Sitefinity 4.1 build version 4.1 1339, thanks for clarifying how to properly reproduce the mentioned behavior. I have just tested the same functionality with build version 1210 (SP1) and the only scenario where this behavior reproduces is if you explicitly specify Document Mode : IE9 standards (the page default is IE8 standards). However this issue has been fixed with 4.1, as you saw on the provided video. Like you have properly observed SP1 was released almost two weeks before the official release of IE9, which might have caused the omission of this issue, which was fixed later. Thank you for the detailed feedback and patience while exploring this issue.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items