Just kinda thinking aloud here...

I think the main difference is that with multi-monitor support, some of the monitors can have negative coords. Maybe there's some logic in the built-in functions that try to make sure a window isn't openning off-screen (e.g. no negative coords) that should be re-writen for multi-monitor support.

Although, if that's the case, then it would seem to be an issue of updating the toolkit, not something that you specifically do in an application.