Thank you for getting back to me.
Thank you for recording this video, and showing me how the problem reproduces. However, I'll have to ask you to give me the code of your user control and master page, because I am still unable to reproduce this with a custom button of mine (please see the attached video).
P.S. Concerning attachments, sorry if this caused any inconvenience, these are the limitations to a forum thread. If you open a support ticket, you should be able to zip any *.swf file and attach it through the attach file utility (size limit is set to 20MB), otherwise we can proceed with uploading the files on an external hosting site, if it's not a problem for you.
the Telerik team