Keep non-ASCII startup input as UTF-8

[austinywang]

Fixes #12915

Summary

cmux workspace create --command "printf '\033]0;라마바OSC테스트\007'; sleep 90" gave the workspace a mojibake title, while --name with the same text worked. The OSC decode was fine: the shell never received UTF-8 in the first place. --command (and the other --command creation paths) passes its text to Ghostty as startup input. The embedded apprt escapes startup input with std.zig.stringEscape, which writes each non-ASCII byte as \xNN, and termio parses it back with config/string.zig, which encoded each \xNN as a UTF-8 codepoint. So 라 (EB 9D BC) reached printf as C3 AB C2 9D C2 BC, and the title it printed was UTF-8 read as Latin-1. The workspace label, sidebar, tab title, cmux workspace list, notifications and session restore all show the title Ghostty parsed, so every one of them was wrong.

The Ghostty fork now carries upstream's fix, 29b82dd80c46 ("config: preserve bytes in hex escapes"), cherry-picked in manaflow-ai/ghostty#239, so \xNN is one byte again and startup input reaches the shell unchanged. Any non-ASCII text passed with --command is fixed, not just OSC titles.

Regression window. The issue points at 0.64.25, but nothing between v0.64.24 and v0.64.25 touches this path. The Ghostty escape/parse mismatch is older; it became user-visible when --command shipped in #9614 (v0.64.23). Titles set with --name, and titles printed by commands typed into a shell, never went through startup input, which is why they looked fine.

Trade-offs

• A text: keybind with \xNN at or above 0x80 now sends that raw byte rather than the codepoint's UTF-8. That matches upstream and Zig's own string syntax; \u{...} still sends a codepoint.
• The fork branch starts from the current pointer edefce778, so this PR doesn't pull in the newer, unrelated commits on the fork's main. The fork PR is left open for a maintainer.
• An OSC title that isn't valid UTF-8 is dropped by Ghostty, keeping the previous title, rather than shown with replacement characters. That's existing behavior; the new test pins it so such bytes can't come back as a Latin-1 title.

Testing

Red commit 992bf8e adds the test; fix commit 3ec1a57 bumps Ghostty; 566794a pins the GhosttyKit checksum.

• cmuxTests/GhosttyStartupInputUTF8Tests drives the real startup-input path: it creates a surface with the exact \033]0;라마바OSC테스트\007 payload as initial input, checks the bytes the shell would read, echoes them back and asserts the title is 라마바OSC테스트. It repeats this for Japanese, emoji with a ZWJ sequence, and combining marks, and checks that an invalid-UTF-8 title never shows up as Latin-1 and doesn't block the next title.
  • Red on 992bf8e: pending, run 36359955943.
  • Green on 566794a: pending, run 36361490263 (with GhosttyCopyModeScrollbackTests, which shares the test terminal).
• Ghostty tests parse: hex escapes are bytes and cloneParsed restores Zig-escaped bytes passed in GhosttyKit run 36360076795 at e168fd31c (75 tests), which also published the pinned xcframework.
• Dogfood tour on red 992bf8e (run 36360118764) sent the verbatim repro through workspace.create and read workspace.list: the title came back as printf '\033]0;ë¼ë§ë°OSCíì¤í¸\007'; sleep 90, and the window title, sidebar row and tab showed the same mojibake. The after-fix tour on 566794a is pending.
• python3 scripts/verify-local.py: 15/15 static checks passed. python3 scripts/swift_file_length_budget.py passed.
• Tagged app repro on the final SHA: pending.

Not verified: no local native build or test run on this machine (per the machine-load rule); iOS is untouched.

Changelog

Fixed: Non-ASCII text passed with --command, such as a Korean terminal title it prints, no longer arrives in the shell as mojibake

Checklist

• Behavior changes have added or updated tests, or Testing says why not
• UI, settings, menu, schema, help-text or user-facing docs change: localization audited — no user-facing strings changed
• User-facing docs updated if needed — docs/ghostty-fork.md records the fork change

🤖 Generated with Claude Code

---

Summary by cubic

Fixes #12915: non-ASCII text passed with cmux workspace create --command reached the shell as mojibake, so a Korean OSC title printed by the command showed mojibake in the workspace label, sidebar, tab, list, and notifications. Ghostty escaped each non-ASCII byte of startup input as \xNN and termio decoded each escape as a UTF-8 codepoint; the fork now preserves bytes in hex escapes so startup input reaches the shell unchanged. The issue became user-visible when --command shipped (v0.64.23); --name titles and titles printed by commands typed into a shell never went through startup input.

Side effects

• A text: keybind that uses \xNN at or above 0x80 now sends that raw byte; \u{...} still sends a codepoint.
• An OSC title that isn't valid UTF-8 is dropped, keeping the previous title — existing behavior now pinned by a test.

Testing

• New GhosttyStartupInputUTF8Tests drives the real startup-input path with the issue's exact payload, covering Korean, Japanese, emoji with a ZWJ sequence, and combining marks, plus invalid UTF-8.
• The Ghostty parse and round-trip tests now run in build-ghosttykit.yml before packaging.
• Branch merged with main (a616a2b); the only conflict was the pbxproj file, resolved by unioning the added test entries then re-normalizing.

^{Written for commit 7029467. Summary will update on new commits.}

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• .github/workflows/build-ghosttykit.yml
• cmux.xcodeproj/project.pbxproj
• cmuxTests/GhosttyStartupInputUTF8Tests.swift
• cmuxTests/ScrollbackTestTerminal.swift
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