CSS Overflow working vertically, but not horizontally?

Can't tell you how many hours I've spent trying to do this, but it's a lot!
An example page with the issue can be found here..

I'm trying to change it to be a horizontal scroll instead of vertical.

Top image here is what happens when I expand the data in the central box.
Lower image is when it fits.

The piece of CSS which works this is here..

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