Can we have an option to make the dialog always overlay the "top" window?

We have an iframe in our pages which contains our most business contents. If we popup a dialog in iframe, the dialog only overlays the document of iframe but not the whole screen. Is it possible that the dialog provides an option to control if it always covers the top window?

I have tried to call top.$(el).dialog() to make it popup in top window. It makes the dialog content appended to the top window, however the propblem is that the top window doesn't import some important css files which are imported in iframe pages, the dialog content can't be displayed normally, without any style.

It's very important for us to provide better user experience. Thank you the great team very much!