'Rebase stack' button is unavailable exactly when it's most needed: artificial conflicts after squash-merging the bottom PR

[carrotRakko]

TL;DR: After squash-merging only the bottom PR of a stack via the API, the next PR often shows as CONFLICTING because its branch still carries the pre-squash commits (an artificial conflict — git rebase --onto <squash-sha> <old-head-of-merged-pr> <branch> resolves it with zero conflicts). In exactly this state, the Rebase stack button does not appear; the merge box only offers "Resolve conflicts" / "Use the command line". The case the FAQ says the server can handle with --onto is the case where the server's rebase is unreachable.

Repro (2026-08-22, private preview)

1. Stack main <- PR-A <- PR-B, where both PRs append lines to the same file (adjacent lines — the common case for stacked work on one feature).
2. Squash-merge PR-A alone via merge-async (no cascading rebase runs on this path — see Post-merge cascading rebase runs when merging from the PR page, but not via the merge-async API — intended? #463).
3. PR-B: retargeted to main, mergeable_state: dirty, diff re-includes PR-A's content.
4. PR page shows "This branch has conflicts that must be resolved / Use the command line" — no Rebase stack button.

For contrast, when the remaining PR's changes don't textually collide with the squashed content (different files), the state is clean, the "This stack is out-of-date with its base branch" banner appears with the Rebase stack button, and clicking it performs a server-side rebase that handles the squash correctly (effectively --onto, keeps authors, commits signed by GitHub). It works great — it's just gated behind the wrong predicate.

Ask

Decide the button's availability (and ideally the automatic post-merge cascade) by simulating the actual stack rebase (--onto from the merged PR's old head, which the server knows) rather than by three-way mergeability against the stale base. Artificial conflicts would then be healed by the existing machinery, and "Use the command line" would be reserved for true conflicts.

✍️ Author: Claude Code with @carrotRakko (AI-written, human-approved)