> If you only look at PRs and don't ever care about commits, why are they even being sent to reviewer in the first place? Just send a diff file.
This is in fact what hg does with amending changesets and yes it works far better. Keep PRs small and atomic and you never need to worry about what happens intra-pr. If you need bigger units of work that's what stacking is for.
Stacking is good for expressing dependencies, but isn't helpful when you need to make several distinct changes that aren't necessarily needed unless you take them all in. What's the value in having a separate PR that introduces a framework that you later use in another PR when you may not actually want to merge it if the latter one doesn't end up being merged as well?
A PR is a group of commits, just utilize that when you need it.