When there are extra-wide inline images in the message body, it stretches the message area to fit the image and makes it hard to read the text in the message (you'd have to move the scrollbar back and forth to read each line). This aims to work around that by forcing the message area to a reasonable fixed width (800 px currently). If any images are too wide to display in that width, a scrollbar is added to the message area that you can drag to see the whole image (but at least you don't have to do that to read the text!).

See http://userscripts.org/scripts/show/67392 for a better solution.