PR timeline renders "deleted the branch <default>" after a stacked-PR base auto-retarget, although the default branch was never deleted

[m3di]

🏷️ Discussion Type

Bug

Body

What happened

In a private repo I had two stacked pull requests:

• PR A — head feature-2 → base development
• PR B — head feature-3 → base feature-2 (stacked on PR A)

PR A was merged into development with "Automatically delete head branches" enabled, so feature-2 was auto-deleted. GitHub then auto-retargeted PR B's base from feature-2 to development (automatic_base_change_succeeded) and auto-closed PR B. I reopened it.

PR B's timeline then shows an event reading " deleted the branch development" — implying the default/protected branch development was deleted.

Root cause (from the timeline API)

The underlying event is base_ref_deleted, and that event object carries NO ref/branch field at all (keys: actor, commit_id, commit_url, created_at, event, id, node_id, performed_via_github_app). The sequence, one second apart:

05:43:59Z base_ref_deleted (real deletion: the OLD base / feature branch)
05:44:00Z automatic_base_change_succeeded (base retargeted feature-2 -> development)
05:44:00Z closed

Because base_ref_deleted stores no historical ref, the UI renders "deleted the branch X" using the PR's CURRENT base ref — which the auto-retarget had just changed to development. So the label names a branch that was never deleted. The branch actually deleted was the old base (feature branch).

Why it's a problem

development is the default, protected branch. A "deleted" event for it is alarming and triggered a needless incident check. The bug: base_ref_deleted is rendered against the live base ref instead of the historical deleted ref.

Expected

Name the branch actually deleted (the old base), or describe the auto-retarget without a "deleted the branch " line for a branch that still exists. Never claim a branch was deleted when it's still present.

Reproduction

1. PR A: head feature-2 → base development.
2. PR B stacked: head feature-3 → base feature-2.
3. Merge PR A into development with auto-delete-head-branch (deletes feature-2).
4. GitHub auto-retargets PR B to development and auto-closes it.
5. Reopen PR B, inspect timeline → "deleted the branch development".

Evidence the base branch was NOT deleted

• git ls-remote origin development → live post-merge commit (branch exists).
• Branches API → exists, protected: true.
• Repo metadata → development is still the default branch.
  Only the old-base feature branch was deleted by the auto-delete cleanup.

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