ModalPopup Extender is covered partially by JQuery.Layout

I designed a page, which is layouted by Jquery.layout. and I put a label in one of its panes.  the label is attached a ModalPopup Extender. (I found ModalPopup extender has some special characterisics which jquery.ui.dialog doesn't. that's why I like to use ModalPopup Extender).

The Issue is : the shown ModalPopup is partially covered by the resizer bar of jquery.layout. I tried to set the z-index of ModalPopup, but it didn't work.

Is there any way to set the ModalPopup shown on Topest layer of a page?

Thanks!