Reject conflict resolutions that bring back lines the parent removed

tests/test_conflict_resolution_resume.sh

@@ -306,5 +306,39 @@ grep -q -- "--base" "$CALLS" && fail "J: base must NOT be edited"
 [[ "$(git -C "$ORIGIN" rev-parse child)" == "$CHILD_BEFORE" ]] || fail "J: child was pushed"
 ok "J: unresolved head re-posts the conflict comment and keeps the label armed"
 
+# ---------------------------------------------------------------------------
+echo "### Scenario K: head merged the target but not the old base's tip -> re-post, keep the label"
+setup_repo
+# The parent got a review commit after the child forked, removing a line. The
+# user resolved with a plain merge of main: the head contains the squash commit
+# but not the parent's tip, and the removed line is back in the child.
+git checkout -q parent
+echo kept > p.txt && echo removed-in-review > r.txt && git add p.txt r.txt && git commit -qm "parent"
+git push -q origin parent
+git checkout -q child
+git merge -q --no-edit parent
+git checkout -q parent
+git rm -q r.txt && git commit -qm "review: remove line"
+git push -q origin parent
+git checkout -q main
+git merge -q --squash parent && git commit -qm "Squash merge parent"
+SQUASH2=$(git rev-parse main)
+git push -q origin main
+git checkout -q child
+git merge -q --no-edit main
+git cat-file -e child:r.txt || fail "K: setup must bring the removed line back"
+git push -q origin child
+MOCK_LABELS="autorestack-needs-conflict-resolution"
+PR_BASE="parent"
+MOCK_COMMENTS_FILE="$WORK/comments.txt"
+{ echo "### conflict"; echo; marker parent main "$SQUASH2"; } > "$MOCK_COMMENTS_FILE"
+run_resume
+
+grep -q "EXIT=1" "$WORK/out.log" || fail "K: run must fail while the PR is unresolved"
+grep -q "gh pr comment" "$CALLS" || fail "K: conflict comment not re-posted"
+grep -q "remove-label" "$CALLS" && fail "K: label must NOT be removed"
+grep -q -- "--base" "$CALLS" && fail "K: base must NOT be edited"
+ok "K: plain merge of the target is not accepted as a resolution"
+
 echo
 echo "All conflict-resume tests passed 🎉 ($PASS scenarios)"

update-pr-stack.sh

@@ -326,19 +326,24 @@ continue_after_resolution() {
     fi
 
     # The user resolved by re-parenting (the comment's `git-merge-onto`), so the
-    # head now contains the squash commit. Verify that and finalize -- do NOT re-run
-    # the merge. Its forced base is the old parent, where the lines the user just
-    # resolved still differ from the trunk, so a re-merge would re-raise the very
-    # conflict they fixed. A plain ancestry check is all the resume needs.
+    # head now contains the squash commit and, through --absorbed, the old base's
+    # tip. Verify that and finalize -- do NOT re-run the merge. Its forced base is
+    # the old parent, where the lines the user just resolved still differ from the
+    # trunk, so a re-merge would re-raise the very conflict they fixed.
+    #
+    # The squash commit alone is not enough: a plain `git merge origin/<target>`
+    # also brings it in, but merges from the natural merge base, which predates
+    # the old base's later commits. Lines those commits removed come back.
     run git update-ref SQUASH_COMMIT "$SQUASH_HASH"
     run git checkout "$PR_BRANCH"
-    if ! git merge-base --is-ancestor SQUASH_COMMIT "$PR_BRANCH"; then
-        # The head does not contain the squash commit: the user pushed without
-        # finishing the re-parent, or followed an old-format conflict comment (a
-        # prior action version) whose resolution never folds the squash in. Re-post
-        # the current instructions and keep the label on so the next push resumes.
-        # Still return 1: a red run flags that the PR is not resolved yet.
-        echo "⚠️ '$PR_BRANCH' does not contain the squash commit yet; re-posting the conflict comment." >&2
+    if ! git merge-base --is-ancestor SQUASH_COMMIT "$PR_BRANCH" \
+        || ! git merge-base --is-ancestor "origin/$OLD_BASE" "$PR_BRANCH"; then
+        # The user pushed without finishing the re-parent, resolved with a plain
+        # merge, or followed an old-format conflict comment (a prior action
+        # version). Re-post the current instructions and keep the label on so the
+        # next push resumes. Still return 1: a red run flags that the PR is not
+        # resolved yet.
+        echo "⚠️ '$PR_BRANCH' does not contain both the squash commit and the tip of '$OLD_BASE' yet; re-posting the conflict comment." >&2
         post_conflict_comment "$PR_BRANCH" "$OLD_BASE" "$NEW_TARGET" "$SQUASH_HASH" "$PR_NUMBER"
         return 1
     fi