SirPrimalform: Hmm... it doesn't seem to agree very nicely with Chrome and Tampermonkey on my system. I've now got a horizontal scroll bar permanently at the bottom of every GOG page...
Hey, I hadn't noticed it before. I get the scroll bar in both Firefox and SWare Iron and it vanishes by just disabling the userstyle, so I don't think that it's got anything to do with your system, Chrome or Tampermonkey.
Hmm, wonder what in that simple code causes the page to get double in width. I'll try to do some research over the weekend and see if I can come up with something.