If it were 100% wide on 4K, how would your eyes scan the text? It's useful for illustrations (graphics), but I haven't met a person yet who can read wide text as efficiently as one or more narrow columns.
I've given up completely on landscape for monitors. I've rotated my two monitors to portrait.
It works for websites, but it's also ideal for programming (no more looking through a letter box at the code), reading documentation and pdf pages.
I've gone for main in landscape and portrait one on the left - the only issue with portrait can be the height, I find myself having my browser starting half way down, with the top section reserved for things like a console window that aren't being looked at constantly (would be great to be able to fullscreen video on just that bit too).
What type of panels and implementations (monitors sold) work well in portrait mode? I've tried it once and I saw an odd pattern that wasn't very comfortable to look at.
TN panels are usually horrible, as they have (in landscape mode) a very limited viewing angle up or down before colours start to shift or parts of the screen appear way darker than others.
IPS panels should work without a problem in both landscape and portrait mode.
I use IPS and VA panels, but I wouldn't be able to tell what panels it was when I had the bad experience. I'll try it again, but not every IPS or VA panel is good either.
A triple monitor setup with 2 side monitors in portrait mode seems to be optimal these days.
That way your primary monitor can still be used to watch media and play games in the "standard" widescreen format, but your code and browser can readily be viewed on the sides.
I've seen this claim repeatedly but it doesn't match my personal experience. I've taken to using a user stylesheet that disables max-width on all elements. It makes my subjective web experience a lot nicer.
