On the Add new form page there is the floating boxes on the right side, where we can choose the form elements. For the first time the sidebar's bottom is hidden, because the left side is too short and the right sidebar is now position: absolute. If I scroll down the page the right sidebar will be position: fixed, so the block is now visible. Or if the form have more elements, the right sidebar will be visible also.
I have found the css style which is responsible for this: #wpbody-content. I have set it to overflow: visible !important, so now the block is visible.
I've tested it on Mac, with FF and Chrome.