Designing for Dual Screen and Foldable Devices with CSS (2023)

There is a key difference between your laptop and your phone, in that you are usually holding your phone. I imagine UX tweeks for wide devices in the hand that could be useful, but mainly for games and such. Keeping controls to the two sides for if held in both hangs, rather than at the bottom for a more standard portrait screen, and so forth, but these things have already been done.

Other changes for the screen getting wider I agree would be best done for all devices. If you don't want you main layout to go beyond a certain width, maybe jump to two columns, or move footnotes up to the otherwise empty space beside the text that refers to them, or images that would be under a paragraph to its side, …

Just letting the user choose would be my preference. I have a large main monitor (32", with a 24" 1080x1920 portrait screen to the side) given how close my face is to it much of the time, and while it is sometimes (two document pages side-by-side, games, videos) used as one large screen most of the time I use it as a flexible multi-screen (50/50 for two apps, or 33/66 or 66/33, or occasionally split three ways, sometimes split all over the shop like a Mondrian painting for a full-screen terminal session). I feel it would be nice if a phone UX allowed at least two apps side-by-side, or single apps allowed me to split views that way sometumes and sometimes not.

It would be good if there was an OS level setting for something like prefer-unified-display in the same way we have things like prefer-reduced-motion. This should be a choice.