Enable alternate screen scrollback (#2334)

[austinywang]

Summary

• update Ghostty so alternate screens inherit the configured scrollback limit instead of forcing zero
• add a Ghostty terminal behavior test for alternate-screen scrollback
• rebase the Ghostty change on top of the current fork pin fe972c095 and publish/pin xcframework-f66517a01ebdefefdbb77841925d132c44bf1e79

Closes #2334

Verification

• zig build -Demit-xcframework=true -Demit-macos-app=false -Dxcframework-target=universal -Doptimize=ReleaseFast
• GHOSTTYKIT_OUTPUT_DIR=/tmp/ghosttykit-f665-verify ./scripts/download-prebuilt-ghosttykit.sh
• Local tests not run per repo policy; CI should run them.

---

Note

Low Risk
Documentation and prebuilt archive checksum pins only; terminal behavior change is isolated to alternate-screen scrollback in the fork submodule.

Overview
Documents and ships the merged Ghostty fork pin fd8b1da51, which combines alternate-screen scrollback (5579fa80f / f66517a01) with the current cmux main pin f78189ac1, so TUIs in mode 1049 keep scrollback instead of zero.

docs/ghostty-fork.md updates the pinned head, release tag URL, branch (issue-2334-alt-screen-scrollback), a new §13 for the Terminal.zig scrollback behavior, June 2026 merge notes, and a rebase note to preserve alternate-screen scrollback in src/terminal/Terminal.zig.

scripts/ghosttykit-checksums.txt adds SHA→archive checksum lines for the scrollback and merge commits, including fd8b1da51 for the new prebuilt xcframework-fd8b1da51....

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Project	Deployment	Actions	Updated (UTC)
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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

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Run ID: ffd8141b-4d50-4e20-aa46-cd707d59ea45

📥 Commits

Reviewing files that changed from the base of the PR and between 5302202 and c401ac5.

📒 Files selected for processing (3)

• docs/ghostty-fork.md
• ghostty
• scripts/ghosttykit-checksums.txt

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-2334-alt-screen-scrollback

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[greptile-apps]

Greptile Summary

This PR bumps the Ghostty submodule from f78189ac1 to fd8b1da51, a merge of the alternate-screen scrollback patch (f66517a01) onto the current cmux main fork pin. The change makes alternate screens inherit the primary screen's scrollback limit instead of forcing zero, keeping TUI output scrollable while in mode 1049.

• ghostty submodule: pointer advanced to fd8b1da51618a77a5d2bd353af7694dfcfb84c5d, consistent with docs and checksums.
• docs/ghostty-fork.md: adds section 13 describing the scrollback change, a per-file conflict note for src/terminal/Terminal.zig, and June 6 merge notes; updates the current-head and xcframework release-tag references throughout.
• scripts/ghosttykit-checksums.txt: adds four new SHA→SHA-256 rows covering the intermediate commits (f66517a01, 395d13a76, 5579fa80f) and the new fork head (fd8b1da51).

Confidence Score: 5/5

Safe to merge — the submodule SHA, documentation, xcframework release tag, and checksums file are all mutually consistent; the download script will correctly resolve the new pin.

All three changed files agree on the new Ghostty fork head fd8b1da51: the submodule points to the full SHA, the doc references that same SHA in the release-tag URL and reachability section, and ghosttykit-checksums.txt contains the corresponding SHA-256 row that the download script will look up at build time. The per-file conflict note for Terminal.zig requested in prior review is now present. No logic, auth, or data-path changes are introduced.

No files require special attention.

Important Files Changed

Filename	Overview
docs/ghostty-fork.md	Documentation correctly updated: current fork head, xcframework release tag, section 13 for alternate-screen scrollback, per-file Terminal.zig conflict note, and June 6 merge-conflict notes all match the new submodule SHA.
ghostty	Submodule pointer advanced from f78189a to fd8b1da51618a77a5d2bd353af7694dfcfb84c5d; consistent with docs and checksums entry.
scripts/ghosttykit-checksums.txt	Adds checksum rows for f66517a01, 395d13a76, 5579fa80f (intermediate SHAs), and the new pin fd8b1da51; the download script will find the pinned SHA-256 for the new submodule HEAD.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[ghostty submodule\nfd8b1da51] -->|SHA read by| B[download-prebuilt-ghosttykit.sh]
    B -->|lookup SHA in| C[scripts/ghosttykit-checksums.txt\n+ fd8b1da51 → SHA-256]
    C -->|expected checksum| D{Checksum match?}
    B -->|download from| E[GitHub Releases\nxcframework-fd8b1da51...-crashsubdir-cmux-crash-v1]
    E -->|actual checksum| D
    D -->|✓ match| F[GhosttyKit.xcframework\nalternate-screen scrollback enabled]
    D -->|✗ mismatch| G[Build fails]

    subgraph Fork composition
        H[f78189ac1\ncmux main pin] -->|merge| A
        I[5579fa80f / f66517a01\nalt-screen scrollback] -->|merge| A
    end

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