Forktender and anyone else affected by this I have another small work around.
If you are using chrome, hit f12 in your browser and select the console tab. Paste this in and hit enter:
$(".p-body-main").removeClass("p-body-main")
This somewhat fixes the issue, but you have to do it on every page turn. It's much easier to just run in a smaller browser. I'm just trying to demonstrate what the css problem with the site is.
This should also make it clear to the site developers what the problem is.
The p-body-main css class conflicts with the p-body-main--withSidebar block sizing. Probably due to using min-width instead of max-width on p-body-inner.
Hope that helps, but this is 100% a site css conflict problem and not anyone's individual settings (outside their desktop resolution and browser window size).