Ah interesting, that explains a lot to me actually. Are these intermediary non-portable wheels guaranteed to exist afterwards for anything installed via a modern pip, even if it uses something other than setuptools? (Like something else commonly specified via pyproject.toml as build system, I’m not up-to-date on other existing alternatives)
Because if these non-portable wheels always exist after first install maybe that could solve build isolation not working for p4a: I’m pretty sure our patching is needed only for building these wheels in the first place, so if pip doesn’t rebuild them from scratch then it could just work. Hence I’m then wondering: 1. are these wheels stored in the site-packages or somewhere else? 2. does pip already do wheel caching and will reuse them if they’re in the right spot? If yes I suspect if we set the paths right build isolation could actually work already. In that case my make-disabled-build-isolation-work-better idea could be entirely unnecessary - only if I am guessing right of course
It did actually for me at least 
thanks a lot for the explanations and your patience, this all sounds very helpful
Sorry I don’t fully follow where this particular idea is heading, what problem are you trying to help us solve?