Add display:block to the editor iframe

Add display:block to the editor iframe

Pressing the down arrow key will sometimes scroll the parent element of the iframe, since it generates an inline box with some whitespace below. This parent element having overflow: hidden meant that it was impossible to scroll deliberately and I would be stuck with a white gutter on the bottom of the viewport for the remainder of my editing session.

Instead of overflow: hidden, give the iframe display: block to avoid generating an inline block.