The first draft of the fix called `chunk.decode("utf-8")` directly on
each 4096-byte `os.read()` result, which corrupts output whenever a
multi-byte UTF-8 character straddles a read boundary:
* `UnicodeDecodeError` fires on the valid-but-truncated byte sequence.
* The except handler clears ALL previously-decoded output and replaces
the whole buffer with `[binary output detected ...]`.
Empirically: 10000 '日' chars (30001 bytes) through the wrapper loses
all 10000 characters on the first draft; the baseline TextIOWrapper
drain (which uses `encoding='utf-8', errors='replace'` on Popen)
preserves them all. This regression affects any command emitting
non-ASCII output larger than one chunk — CJK/Arabic/emoji in
`npm install`, `pip install`, `docker logs`, `kubectl logs`, etc.
Fix: swap to `codecs.getincrementaldecoder('utf-8')(errors='replace')`,
which buffers partial multi-byte sequences across chunks and substitutes
U+FFFD for genuinely invalid bytes. Flush on drain exit via
`decoder.decode(b'', final=True)` to emit any trailing replacement
character for a dangling partial sequence.
Adds two regression tests:
* test_utf8_multibyte_across_read_boundary — 10000 U+65E5 chars,
verifies count round-trips and no fallback fires.
* test_invalid_utf8_uses_replacement_not_fallback — deliberate
\xff\xfe between valid ASCII, verifies surrounding text survives.
When a user's command backgrounds a child (`cmd &`, `setsid cmd & disown`,
etc.), the backgrounded grandchild inherits the write-end of our stdout
pipe via fork(). The old `for line in proc.stdout` drain never EOF'd
until the grandchild closed the pipe — so for a uvicorn server, the
terminal tool hung indefinitely (users reported the whole session
deadlocking when asking the agent to restart a backend).
Fix: switch _drain() to select()-based non-blocking reads and stop
draining shortly after bash exits even if the pipe hasn't EOF'd. Any
output the grandchild writes after that point goes to an orphaned pipe,
which is exactly what the user asked for when they said '&'.
Adds regression tests covering the issue's exact repro and 5 related
patterns (plain bg, setsid+disown, streaming output, high volume,
timeout, UTF-8).
