A "New Window" script step [FMSA 7.03 Win/Mac] consistently gives me the contents of that alleged new window in the existing browser window. If I put a "Close Window" button in that window, however, that new content disappears/closes to reveal the referring page content beneath it. But if you use the browser's Close Window, everything closes, just as you would expect.
Summary: On either Mac or Win FMSA, using IWP, I have never seen a New Window script step produce a new browser window.