Fix of overlaid modal window

If top is set to 25% on some screens it overlays the page at the very
top. As it hidden area - page become unclickable at top of page.

I'm prefer to set top: 100% to avoid this (please find attached screenshot):

![Screen Shot 2013-04-28 at 3 17 03 AM](https://f.cloud.github.com/assets/1686778/435611/c622c1dc-afee-11e2-96f3-d68d5c036952.png)

Send to 3.0.0-wip in correspond of #7708 issue