Empty boxes stop pushing pages down

Two small fixes that each close a gap we measured against the reference browser. Many site templates leave a few empty boxes in a row (an empty menu, a placeholder list), each with some spacing around it. The reference browser merges all of that spacing into a single gap; our engine added it again for every box after the first, so everything below slid down. On the encyclopedia's plain version the gap under the search box is now exactly the reference browser's. And scripts that ask how tall a line of text is now get the real size in pixels, the same answer the reference browser gives.