Safari is working as the layout is intended with one small rendering issue. It seems when the page is loaded with content taller than the height of the window, a gutter of about 1em (perhaps the height needed for a horizontal scrollbar?) appears on the bottom-layer color mask. When you manually resize the window, this gutter space disappears.
Anyone know about such a Safari bug? I’ve seen similar under IE5 and usually that called for either position:relative or holly hack.