ive always said that going from one screen to two is a big jump in productivity. Comparing things between two windows is a very common task in almost all workloads. However I think three is already too many, and brings something between barely any benefit and a net negative. More than that seems superfluous, even for cctv or stock brokers. Attention can't be split that easily. Personally for most of my working life ive had three, as in two 24" and a laptop, but I usually either just have spotify fullscreen on the laptop all day or turn it off if I can.
As for putting two windows side-by-side on a single screen? I don't know, it always felt clunky to me. A lot of things are designed to be landscape 16:9.
As for putting two windows side-by-side on a single screen? I don't know, it always felt clunky to me. A lot of things are designed to be landscape 16:9.