Thank you for contacting us.
Since I was able to use two of the approaches from the threads you have mentioned that are working fine in IE, Chrome, Firefox 22.0 (last official version) and Firefox 23.0 beta, I will kindly ask you to provide a code snippet with how you are trying to implement this in your project.
I am attaching a simple project, demonstrating how you can achieve this behavior of the RadTextBox.
Please try it and see if it works on your side.
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