Comment 20 for bug 210241

Don't wish to spam this bug, but my suspicion was very quickly confirmed; this bug doesn't appear when a wmode is set for flash. "Windowless mode" does sound like a good reason for there to be no window associated with Flash. Of course, Flash 9 for Linux doesn't have wmode support, which is why it happens there with every instance of Flash using Flash 9, and only some with Flash 10, and why specifically that Beta is the one where the behaviour changes (wmode was only just introduced for Linux in that Beta).

For what it's worth, try this page on for size, with Flash 10, to see the behaviour:

http://www.communitymx.com/content/source/E5141/wmodetrans.htm