A launchd/Finder-launched server has no LANG/LC_* at all, so tmux — which
decides per client whether the terminal is UTF-8 capable purely from those
variables — fell back to its non-UTF-8 mode and rewrote the byte stream
server-side, before it ever reached xterm:
- every wide character became `_` (中文测试 → ________)
- `✓` / `⏺` / emoji became `_`
- `═║╔╗` was downgraded to DEC Special Graphics (ESC ( 0), so rounded
corners vanished and boxes rendered as loose horizontal lines
The `-l` login shell cannot fix this: tmux is the PTY's root process and has
already made its decision by the time the shell sources ~/.zprofile.
withUtf8Locale() fills a locale in on the spawn env, and only when needed —
an existing UTF-8 LC_ALL / LC_CTYPE / LANG (zh_CN.UTF-8, ja_JP.UTF-8, …) is
left untouched; only a missing or non-UTF-8 charset is replaced.
Existing tmux sessions do not need recreating: re-attaching with a UTF-8
client restores correct output (already-scrolled `_` stays mangled).
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