It happens to me on this forum as well although I am curious that Steve reports earlier in this thread that it did not happen to him. The only difference I know of is that he uses Windows and I use OSX.
I believe this is related to the problem of using the title bar to drag the popup window around. Am I right in assuming that when you try that on your site the window jumps down the screen? We believe this to be a jQuery UI issue but are still looking hard at it to see if we can pin it down.