What do you do if CI is slow and you're merging small commits frequently? For example, what if CI takes 15 minutes and you're trying to merge more than 4 PR's per hour? Our integration tests spin up and reboot Windows machines, it would be tough to get that test under 15 minutes.

We use the "merge when pipeline succeeds" button on GitLab, but if two people have clicked that button it's a race. The winner gets their PR merged, the loser gets their pipeline invalidated because `main` has changed; they have to rebase onto main and try again.

OTOH, if instead of rebasing onto main when running the final pipeline before merge you rebase onto what will be main once its pipeline succeeds your own pipeline will still be valid.

In other words, by using stacking we can do final merge CI pipelines in parallel rather than series.

Unfortunately, we haven't found good tooling for this flow that works with GitLab. git town does stacks and works with GitLab, but it has a few limitations that make it a poor fit for what I call "merge trains".