Stacked PR wipes approvals from the stack

[theblixguy]

🏷️ Discussion Type

Bug

Body

From https://github.blog/changelog/2026-10-06-stacked-pull-requests-generally-available/

Approvals stay in place for unchanged code. When an otherwise unchanged stack is updated after its base branch, such as main, moves ahead, Rebase stack now preserves approvals, even in repositories that dismiss stale approvals.

Assume we have the following settings on:

1. Dismiss stale pull request approvals when new commits are pushed
2. Require approval of the most recent reviewable push

Now let's imagine this situation:

1. I enqueue a stack of 10 PRs into the merge queue
2. The first 5 of them successfully merge
3. The remaining 5 fail (e.g. 6th PR gets kicked out due to a flaky test, which kicks out all the pending PRs)

When something like this happens the base branch of the 6th PR is updated to main (I see a "The merge-base changed after approval" message) and all approvals on that PR (along with approvals on the remaining 4) are wiped, even though there's no actual change to the diff, just the base branches got rebased and updated.

I don't think the approvals should be wiped in this case, at least for stacks and even with those settings on, as it's a bit counter-intuitive.

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