There is something strange with popup control... if used with a ContentUrl
There is always an horizontal scroll bar at the bottom of the popup!
You can have a look on the two screen caps joined!
Even after having made ALL the controls invisible... there is still a scroll bar with an empty popup!
I think that this is not the ASPxPopupControl's scrollbar since there is a button that is eventually residing inside the ASPxPopupControl, but is positioned below the scrollbar. Anyway, you can determine what this element is by using the Firebug plugin for the Mozilla Firefox.