However I suppose also that most is not all, and there must be some outliers who do it on a single laptop screen either from necessity or preference. I might find myself doing extremely narrow windows out of necessity but never preference, and then the extremely narrow windows would almost always be on something I was working on - as it is indeed in your case - and not to interact with someone else's site.

I tend not to read at the same time as programming, I might read, stop reading, and then program based on what I read, but not have two tools open and have my eyes continually hopping between them.