fix(rebase-stack): only delete merged branch after full stack success

Prevent closing child PRs by ensuring the merged branch is only deleted
after all rebases, force-pushes, and base updates succeed. On failure,
the branch is kept and only a comment is left on the affected PR.

Author: behinddwalls

.github/workflows/rebase-stack.yml

@@ -18,7 +18,9 @@
 #      rebase silently altered code, it refuses to force-push.
 #   5. If a rebase hits conflicts, it leaves a comment with manual fix
 #      instructions and stops processing that chain.
-#   6. Deletes the merged PR's head branch (replaces GitHub's auto-delete).
+#   6. Deletes the merged PR's head branch after all child PRs have been
+#      retargeted and rebased. If the chain failed, the branch is kept
+#      to avoid closing child PRs whose base was not yet updated.
 #
 # Why "rebase --onto" instead of "--fork-point":
 #   GitHub Actions runs on a fresh clone with no reflog, so --fork-point
@@ -152,10 +154,6 @@ jobs:
           )
                 gh pr comment "$pr_number" --body "$comment_body"
 
-                # Update the base even on failure so GitHub shows the correct
-                # target branch, even if the diff is still wrong.
-                gh pr edit "$pr_number" --base "$new_pr_base"
-
                 echo "::warning::Stopping chain at PR #${pr_number} due to conflicts."
                 return 1
               fi
@@ -183,7 +181,6 @@ jobs:
 
           Please rebase manually and verify the changes are correct."
 
-                gh pr edit "$pr_number" --base "$new_pr_base"
                 return 1
               fi
 
@@ -198,7 +195,6 @@ jobs:
 
           The rebase succeeded but force-push failed for \`$pr_branch\`. This may be due to a concurrent push. Please rebase manually."
 
-                gh pr edit "$pr_number" --base "$new_pr_base"
                 return 1
               fi
 
@@ -235,15 +231,16 @@ jobs:
             "$MERGED_BASE" \
           || rebase_result=$?
 
-          # Delete the merged PR's head branch. This replaces GitHub's
-          # auto-delete-head-branch setting, giving us control over timing
-          # (we delete only after the stack is rebased).
+          # Delete the merged PR's head branch only if the rebase chain
+          # succeeded. If it failed, some child PRs may still have this
+          # branch as their base — deleting it would cause GitHub to close
+          # those child PRs.
           echo ""
-          echo "Deleting merged branch: $MERGED_HEAD"
-          git push origin --delete "$MERGED_HEAD" 2>/dev/null || echo "Branch already deleted."
-
           if [ "$rebase_result" -eq 0 ]; then
+            echo "Deleting merged branch: $MERGED_HEAD"
+            git push origin --delete "$MERGED_HEAD" 2>/dev/null || echo "Branch already deleted."
             echo "=== All stacked PRs rebased successfully ==="
           else
+            echo "Keeping merged branch '$MERGED_HEAD' to avoid closing child PRs whose base was not updated."
             echo "=== Rebase chain stopped due to conflicts ==="
           fi