This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
By definition, a MODAL window is supposed to prevent the user from using other windows until they are finished interacting with the modal window. Why would you want to usurp that? If you don't want it to be modal, don't use a modal window. You probably need to rethink your user experience design if you feel you really need to open another window from a modal window.