An upstream PR cannot use a fork branch as its base, even with write access to both repos

### Problem

Stacked PRs require every branch in a stack to live in the same repository — [the docs](https://docs.github.com/en/pull-requests/reference/stacked-pull-requests) state: *"Stacked pull requests require all branches to be in the same repository. Cross-fork stacks are not supported."*

**This is not a permissions problem.** I have write access to both the upstream repository and my fork. The blocker is the fork boundary itself: even with full access on both sides, a stack cannot start in a fork and land in upstream.

---

### The workflow that does not work

1. Fork `upstream` → `origin`.
2. Create `feature-a` and push it to `origin`.
3. Open a PR: `origin:feature-a` → `upstream:main`. This works today — it is an ordinary cross-fork PR.
4. Stack `feature-b` on top of `feature-a`, push it to `origin`.
5. Now I want that second PR to join the stack of the PR from step 3, in `upstream`.

Step 5 has nowhere to go:

- A PR opened in `upstream` must have a base ref that exists in `upstream`. There is no way to express *"base = `origin:feature-a`"* for a PR that lives in `upstream`, so the upper layers of the stack have no home in the repository where the stack needs to be reviewed.
- Opening `origin:feature-b` → `origin:feature-a` instead puts that PR in the **fork's** PR list, split off from the step-3 PR. The two are not one stack, and upstream reviewers never see the layering.

So the stack I want is:

```
PR #3: origin/feature-c → origin/feature-b ┐
PR #2: origin/feature-b → origin/feature-a ├─ one stack, reviewed in upstream
PR #1: origin/feature-a → upstream/main ┘
```

and this cannot be created, in either repository.

---

### Incremental stacking on an already-open PR

The second half of this is just as important: the stack is usually **not** planned up front. I open a single cross-fork PR first, keep working, and only later realize the next change belongs on top of it.

So it should be possible to take an already-open cross-fork PR (`origin:feature-a` → `upstream:main`) and attach a new layer to it afterwards, turning it into the bottom of a stack — rather than having to decide at creation time that this PR will be stacked, or having to tear down and recreate the existing PR.

---

### Why this matters

Forking is not only the OSS drive-by contribution path — it is also a normal way to work when you *do* have access on both repositories: keeping topic branches out of the shared repo, working across an org boundary, or simply following a project's convention that all branches live in personal forks.

Right now the two features are mutually exclusive. If branches live in a fork, stacked PRs are unavailable; to use stacked PRs, every branch has to be pushed into upstream directly. That forces a choice between the fork-based branch layout and the ability to present work as reviewable layers.

For scale, from [Octoverse 2025](https://github.blog/news-insights/octoverse/octoverse-a-new-developer-joins-github-every-second-as-ai-leads-typescript-to-1/): 63% of all repositories on GitHub are open source or public, and 518.7M pull requests were merged across public and open source repositories in 2025 (+29% YoY). Every project whose branches live in forks is currently outside the reach of stacked PRs.

---

### Proposed behavior

Allow a stack whose branches all live in a single fork, with the bottom-most PR targeting upstream:

```
origin/feature-a → upstream/main
origin/feature-b → origin/feature-a
origin/feature-c → origin/feature-b
```

treated as one stack in the upstream repository, with the existing permission and review model unchanged — each PR is still an ordinary cross-fork PR reviewed and merged by upstream — and no change to the underlying Git branch model.

Supporting stacks that span *multiple* forks (basing work on another contributor's open PR) would be a reasonable later step, but single-fork → upstream covers the common case.

---

### Related reports

The request has come up repeatedly, and has been acknowledged, but has not shipped:

- **#18** (discussion, 2026-04-13) — "Stacked PRs with branches across forks", opened the day the private preview started: *"without this it's not very useful yet."*
- **#46** — "FR: support for cross-fork stacked PRs" (2026-04-16). @skarim replied on 2026-04-17: *"support for Stacked PRs across forks will be coming in a future update! ... it is a high priority for us and on our roadmap"*, describing the single-fork → upstream shape as the first step. Repeated +1s and ETA requests since, with no further update.
- **#509** (2026-09-13) — `gh stack submit` on a fork still fails on **v0.1.1**, resolving the stack against the upstream parent instead of the fork: `Head sha can't be blank, Base sha can't be blank, ... Head ref must be a branch`. `gh repo set-default ` does not change it.
- **#496** — `gh stack link --remote ` queries the fork's parent repository.
- **community#201439** — the public preview announcement thread, where several people raised cross-fork support as the blocking gap.

Five months after the roadmap confirmation, the v0.1.0 (2026-07-29) and v0.1.1 (2026-09-02) release notes contain no cross-fork work, and reports against the current release are still arriving (#509, two days ago).

I am filing this separately from #46 because the existing threads frame cross-fork stacking as an access problem for contributors who cannot push to upstream. The case above is reproducible **with write access to both repositories**, and it also covers attaching a stack to a cross-fork PR that is already open — which I did not see raised elsewhere. Happy for this to be folded into #46 if you would rather track it there.

---

### Ask

Where do cross-fork stacks currently sit on the roadmap, and is there a target release for the single-fork → upstream case? Happy to test a preview build.

Bắt đầu bằng cách tái hiện hành vi cross-fork với gh stack submit và gh stack link --remote, sau đó xem xét các issue liên quan #46, #496 và #509 để kiểm tra các ràng buộc hiện có. Hoàn thành nghĩa là một stack single-fork có thể nhắm đến upstream, bao gồm cả việc thêm một layer vào PR cross-fork đã mở, đồng thời duy trì hành vi thông thường về quyền và review.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.