fluid background image cannot vertical expand to whole screen

Created by: AntiGameZ

I'am using the latest code in dev branch and the problem exists in master.

If a background image added to body, like

body {
    background: url("http://subtlepatterns.com/patterns/littleknobs.png") top left repeat;
    padding-top: 60px;
}

Then delete hero-unit and all rows in examples/fluid.html. You can see bottom of the page is white.