alt Hacker NewsOut of curiosity, do you happen to know why Apple thought that would be the cause for low adoption among 3rd party apps? Isn't scaling something that the OS should handle, that should be completely transparent, something that 3rd party devs can forget exists at all? Was it just that their particular implementation required apps to handle things manually?
I can only offer a hypothesis. Historically UI sizing was done in pixels, which means they are always integers. When developers support fractional scaling they can either update the app to do all calculations in floating point and store all intermediate results in floating point. That's hard. Or they could do calculations in floating point but round to integers eagerly. That results in inconsistent spacing and other layout bugs.
With 2x scaling there only needs to be points and pixels which are both integers. Developers' existing code dealing with pixels can usually be reinterpreted to mean points, with only small changes needed to convert to and from pixels.
With the 2x-and-scale-down approach the scaling is mostly done by the OS and using integer scaling makes this maximally transparent. The devs usually only need to supply higher resolution artwork for icons etc. This means developers only need to support 1x and 2x, not a continuum between 1.0 and 3.0.