nsIDeviceContext::SetCanonicalPixelScale

Hi. I'm using nsIDeviceContext::SetCanonicalPixelScale method to change
scale for my web page. Everything works fine. But after page scale changing
I can not get real coordinates of my DOM objects. Coordinates are old (as
were before scale changing). Does anybody know how I can get the real
coordinates? Or maybe somebody know another way to change page scale?

Thank you,
Michael.