Actually, this is especially true when both are 1080p, because laptop screens are never as big as desktop monitors, and we also tend to use them closer. I have this exact problem right now but I think I've just adjusted my eyes over time to squinting at 1080p at 14", or perhaps I turned on some display scaling and forgot about it.
> For example, you can have a window that straddles both monitors. What should the scaling be?
Intuitively, I feel like you should use the physical DPI of both screens to make sure that the window has the same physical dimensions on both. But that'd probably lead to weird scaling factors like 1.17 instead of nice round ones, and thus fuzzy scaling, so it probably couldn't quite work. I guess perhaps you'd just snap each display's DPI to the closest predefined value (eg. .25 increments which I think most systems use these days). Then you'd get a similar-sized part on both sides of the boundary.
But yeah, I think overall if you actually use physical DPI for scaling everything should work out close to nicely.