Thank you for taking the time to record the video for us.
Indeed, it is a bug and we have opened a bug report for the Modal Window. With your vote already given there, the reported bug priority is increased, and we are tracking it. Also, you can go and click the "Follow" button so you get notified via email about status changes.
The bug occurs only if there is a vertical scrollbar on the page. I am afraid we are not aware of a workaround at this point.
In the attached sample project, you can find an example with the source code from our demo and how it works when there is no scrollbar.
If you have any other questions, please let me know.