It sounds like you're talking about a bunch of different cases, and I'm having trouble untangling them.
If there's a simple refactoring everybody agrees is good whether or not your overall goal ends up making sense, then yes, by all means merge that. But that doesn't require stacking unless your review process is slow. In which case I still think the right solution is to speed up review, not to stack.
For the cases where we don't know the full story, my first thought is that we never know the full story. So there I try to instead find the smallest unit of work that everybody agrees is a step forward.
When that's not possible, where the unit of work still seems pretty large, instead of breaking that up into a bunch of stacked diffs that shouldn't be merged until we really understand something (which to me sounds like a large PR in disguise), I think a better option is a spike, where we intentionally do a quick, throwaway version of some change as a way of learning about the change. Instead of trying to do good code along the way to good understanding, we just go for the understanding. Once we have thrown out that scratch code, we then go back with our new knowledge for a proper PR.
The premise of stacked diffs seems that we won't learn anything significant from reviewing or deploying code. (If we did learn something valuable, then the things stacked on top could be up for a lot of rework or might be thrown out altogether.) I think that has a lot of bad effects, but one of the biggest for me is that the bigger the inventory of code (whether in one big PR or a stacked set of smaller ones), the more a reviewer will feel obliged to say, "LGTM" and let it go, because they know there's not much point in saying, "Actually, I think this whole thing could be better approached by X."
So like you I'm entirely for small, reviewable lumps that are easily merged. I just think they should be then reviewed quickly, so that stacking or agglomerating is unnecessary.
> unless your review process is slow. In which case I still think the right solution is to speed up review, not to stack.
This feels like a critically important point to your position but there's no actionable advice provided on how to achieve that. FWIW I've found stacked PRs do speedup reviews because it pipelines the work. Pipelining something removes bubbles (in this case time spent waiting on review) from forming.
Simpler parts of the work get eyeballs from more junior engineers who feel more comfortable approving smaller / simpler PRs (& other people feel confident in that). Trickier stuff is left to the smaller pool of people who have the appropriate context / skillset. If you're waiting on landing 1 PR at a time, then you're serializing the review flow which means your total time on the PR is "time spent writing + time spent reviewing 100% of the code". If you pipeline your stacked diff, then potentially you could get ~80% of the code reviewed & landed by the time you finish the more complex pieces. Then you're left with "time spent writing + time spent reviewing 20% of the code". Additionally, by putting up the commits early, you're letting other people fit smaller reviews into their schedule more easily vs "here's a PR with 5000 lines of code changes" which is a monstrosity to review (i.e. quickly runs into mental fatigue issues / quality of the reviews can easily degrade, especially if commit hygiene wasn't practiced).
Have you actually tried stack PRs with a proper review process & good commit hygiene? This is one of those "try it before you knock" it things.
Again, I think you're tangling together cases. And also tangling arguments. You can't argue from theoretical gains in an imagined optimal case and then say the only argument that matters is the empirical one.
If your point is that in some existing organizations stacked pull requests are better for a specific engineer's experience than doing all the related code in a big blob, I certainly believe you.
Similarly, there are manufacturing shops where reducing inventory in line with Lean approaches doesn't work out of the gate, because there are other organizational problems/constraints that have to be dealt with first. For example, you might need a large buffer of component X at stage Y of a manufacturing process because upstream quality issues mean that a smaller buffer would cause frequent stalls at stage Y. First you have to fix the upstream issue before you can cut stocks there.
So are stacked pull requests the optimal choice for some specific person on some specific occasion? Sure! I'll take your word that's the case for you. What I'm saying is that I think they're an indicator that there is some systemic problem that could be resolved so stacked pull requests and giant pull requests both become unnecessary.
If there's a simple refactoring everybody agrees is good whether or not your overall goal ends up making sense, then yes, by all means merge that. But that doesn't require stacking unless your review process is slow. In which case I still think the right solution is to speed up review, not to stack.
For the cases where we don't know the full story, my first thought is that we never know the full story. So there I try to instead find the smallest unit of work that everybody agrees is a step forward.
When that's not possible, where the unit of work still seems pretty large, instead of breaking that up into a bunch of stacked diffs that shouldn't be merged until we really understand something (which to me sounds like a large PR in disguise), I think a better option is a spike, where we intentionally do a quick, throwaway version of some change as a way of learning about the change. Instead of trying to do good code along the way to good understanding, we just go for the understanding. Once we have thrown out that scratch code, we then go back with our new knowledge for a proper PR.
So I'm still not seeing where I would use stacked diffs, except in this case here: https://news.ycombinator.com/item?id=32215346
The premise of stacked diffs seems that we won't learn anything significant from reviewing or deploying code. (If we did learn something valuable, then the things stacked on top could be up for a lot of rework or might be thrown out altogether.) I think that has a lot of bad effects, but one of the biggest for me is that the bigger the inventory of code (whether in one big PR or a stacked set of smaller ones), the more a reviewer will feel obliged to say, "LGTM" and let it go, because they know there's not much point in saying, "Actually, I think this whole thing could be better approached by X."
So like you I'm entirely for small, reviewable lumps that are easily merged. I just think they should be then reviewed quickly, so that stacking or agglomerating is unnecessary.