"Merge stack" on a stacked PR appears to enqueue only the bottom PR

[annyzhou]

Summary

When a repository requires GitHub Merge Queue on the trunk branch, clicking "Merge stack" on a stacked PR appears to enqueue only the bottom PR. The UI says the stack can be merged together, but the merge queue only recognizes the PR whose base is the protected trunk branch.

Repro

1. Create a stack:
   • PR A targets dev
   • PR B targets PR A's branch
2. Ensure both PRs are approved, green, and linear.
3. Open PR B.
4. GitHub shows "Able to merge as a stack" and "Merge stack 2".
5. Click "Merge stack".

Expected behavior

Either:

1. The entire eligible stack is merged/queued correctly, or
2. The UI explains that merge queue can only queue the bottom PR and the rest must be merged after each lower layer lands.

Actual behavior

Only the bottom PR is recognized by merge queue. The upper PR remains unmerged and must be merged manually after the lower PR lands.

Why this is confusing

The button says "Merge stack" and shows a count of PRs, which implies the stack will merge as a unit. With required merge queue, the actual behavior is one-layer-at-a-time.

FYI, our merge queue config is fine:

• max_entries_to_build: 5
• min_entries_to_merge: 1
• max_entries_to_merge: 3
• strategy: ALLGREEN