How to get screen position before using BrowserWindowOpener?

I am trying to use BrowserWindowOpener to open a window that is separate from the current browser window.
If I open the window without setting left and top in the features list, the location of the popup is  a mixed bag.  Firefox at least tries to open the window over the application, but IE and Chrome, not so much. 

I can set left and top, but these are in screen space, so to have any control over where the popup comes up, it looks like I need to get the application's location in screen space first so that I can use that to get my window to pop up relative to the application.

This is especially important if the application is opened up on the right monitor of a dual monitor setup.  Setting left and top in this instance can make the window appear off on the other screen.