Why contents of a div are not shown while dragging with the draggable="true"?

So I'm using the `draggable="true"` for a div that holds couple of other divs with `span` classes applied from Twitter's Bootstrap. The problem that raises is when I'm dragging the holder the children elements are not shown. If I remove the classes from Bootstrap then the elements are shown during drag. The broken example can be found on jsfiddle.net/8umGq/. What exactly is causing the problem and what should I probably override in order to use the `span` classes and still see the contents while dragging? I want to make it work on a latest or beta release of Chrome and still preserve style and the positions of the `span` classes.