I've just attached a patch to this issue that sets the page layout to report for all course reports and fixes a couple of minor display glitches in the serenity theme. Could one of the watchers I've added please review these changes.
In regards to the bug itself, wide content is still hard to deal with thanks to the floating divs layout, however we do now have the report layout which in most theme's allows the browser to scroll horizontally.
However using the report layout isn't always the correct solution either, things like tables-for-layout should be converted to div structures and areas where people may attach horrendously large images should introduce there own resizing or overflow solution depending upon the layout of that page and what space there is to play with.
Anyway hopefully this patch will get commit soon and you will once again be able to scroll horizontally on the completion progress report as well as the other course reports if required.