Hi Vicentiu,
If setting the z-index property can not make sense, you can define a valid BackgroundCssClass to the ModalPopup, thus, a new DIV which is set as the back ground would be generated to cover the window.
Best regards,
Zhi-Qiang Ni
Microsoft Online Community Support
Please remember to click “Mark as Answer” on the post that helps you, and to click “Unmark as
Answer” if a marked post does not actually answer your question.