docs(runbook): stale "out-of-date" PR after a stacked PR is retargeted (refs #195)

Gitea 1.27 blocks the merge on a stored commits_behind that a same-second
retarget + force-push can leave stale, while Update branch recomputes it
live and refuses. Record the symptom, the cause and the amend + force-push fix.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/runbooks/gitea-actions-gotchas.md

@@ -15,6 +15,7 @@ those containers share a filesystem — or a `localhost` — breaks.
 | `pg_isready` passes before PostGIS is ready | add a `PostGIS_Version()` probe | the db healthchecks |
 | `upload-artifact@v4` fails ("not supported on GHES") | pin `@v3` | `.gitea/workflows/ci.yaml` (`mutation` job) |
 | `upload-artifact@v3` fails with "Artifact service responded with 500" | mark the upload `continue-on-error: true` (server-side; issue #62) | `.gitea/workflows/ci.yaml` (`mutation` job) |
+| PR says "out-of-date" but *Update branch* fails ("Unable to update pull request") | push a new head SHA (`commit --amend --no-edit` + `push --force-with-lease`) | §10, server-side |
 
 ---
 
@@ -289,3 +290,41 @@ re-run hides the failed one, so keep the failing job id from the original report
 - Remember `concurrency.cancel-in-progress: true` in `ci.yaml`: a new push to the same
   ref, or a re-run, kills the in-flight run the same way. Check `run_attempt` before
   concluding a job hung.
+
+---
+
+## 10. A retargeted stacked PR says "out-of-date" but *Update branch* fails
+
+**Symptom** — the PR shows *This branch is out-of-date with the base branch* and
+*This pull request is blocked because it's outdated* (branch protection
+`block_on_outdated_branch` on `main`), yet **Update branch** answers *Unable to update
+pull request* (API: `HeadBranch of PR NN is up to date`). `git merge-base` confirms the
+branch sits on the tip of `main`. Seen on #194 (#195).
+
+**Why** — Gitea 1.27 keeps two answers to "is it behind?":
+
+- The banner and the merge block read a **stored** `pull_request.commits_behind`
+  (`MergeBlockedByOutdatedBranch`: `CommitsBehind > 0`).
+- *Update branch* recomputes it **live** from git (`services/pull/update.go`) and refuses
+  when nothing is behind.
+
+The stored count is only refreshed by a push to the head or base branch, or by a
+retarget. #194 was stacked on #193; its rebased branch was force-pushed in the **same
+second** that merging #193 deleted the parent branch and Gitea retargeted #194 to `main`.
+The retarget compared `main` against `refs/pull/194/head` before the push queue had
+updated that ref (the old head missed the squash commit → `CommitsBehind = 1`); the push
+handler's own resync ran against the deleted old base and failed. Nothing pushed
+afterwards, so the stale `1` stuck. Close/reopen does not recompute it.
+
+**Fix** — give the head a new SHA with the same content; the push resyncs the count:
+
+```bash
+git commit --amend --no-edit          # new committer date → new SHA
+git push --force-with-lease origin <branch>
+```
+
+PR CI re-runs on the new SHA (the old statuses don't carry over).
+
+**Avoid it** — when the parent of a stacked PR merges, let Gitea finish retargeting the
+child to `main` (its timeline shows *changed target branch*) **before** pushing the
+rebased child branch.