When I put in content that should make the region-content area strech it is cropped off at the end of the screen, rather than growing to fit correctly.

Just put the attached php file into admin/reports/profiler/index.php you will see the error.

I have made that page output a div, table with on column and table with 30 columns all with no width.

I have also output all of the above with a width of 2000px to try and make the page strech.

The content without width fits the page as expected (asside from the table with 30 columns), but the content that has been given a fixed width just gets cut off at the edge of the page.