Thank you for contacting Telerik,
I was able to reproduce similar behavior using the RadControls’ RadDatePicker module. Assuming you are using a similar setup in RadControls, could you please try setting the rcCalPopup class to the DatePopupButton.CssClass and let me know if the issue still exists.
If you aren't using RadControls, please disregard the previous instructions.
In order for me to serve you best, we’ll have to gather more information about your setup. What tools are you using to create the calendar? How often are you experiencing the behavior in Test Studio? Does the Date Picker ever exhibit the behavior outside of Test Studio?
Additionally it will be helpful if you could post your aspx page markup with the correspondent code behind code. Thus we will be able to gather more details about your scenario and provide you with more to-the-point answer.
the Telerik team