Slither of background item at edge of page

Hello,

I often get a sort of boarder around an item that should be on top of the item that is coming through. Like this:

The purple is in the background, behind the white object (with text on top of it). The white object is positioned so that it overlaps the edge of the page by quite a bit. So why do I see a slither of purple on the left? The white object should be covering it.

Thanks.