If a photo description is too long it remains in a single line going off the screen and causes the appearence of a horizontal scrollbar.
A workaround I've used is to add the following to MediaWiki:Common.css.
.fancybox-title-float-wrap .child {
white-space: normal;
}
which has the following result:
The description is now readable but lays over the image.
I guess what needs to be done here is somehow for the frame to auto adjust itself to fit alongside with the description.