Created attachment 57754 [details] Reduction that illustrates the bug See attached testcase. #content should not have the gap on the right, and it doesn't in ALL rendering engines (Trident, Presto, Gecko) but Webkit-based ones. If you remove overflow:hidden the bug disappears. If you remove #sidebar, it disappears again. I have no idea what might be causing it.
It looks the problem between left-float and left-margin. I guess that the "content" width is caclulated as [the containg block's available width of "content"(960-siderbar's width) - left-margin(18em) value] But, margin is the distance from containing block not neighbor block. So, left-margin calculated from left edge of containing block("wrapper") BTW, in case of overflow:hidden, I have two theory. 1. The line box of "content" has right value and draw the bgcolor. But, the line box has no background-color. The painting in webkit draws the inherited background-color from parent? not transparent? 2. Or the block width calculation is wrong only when it has "overflow:hidden" But, "overflow" property should effect on "drawing" not "rendering", isn't it? I'll analize this bug with latest nightly build.
*** This bug has been marked as a duplicate of bug 35910 ***