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fix post-TUI typing lag: latch fish query-term off in zellij pane shells
Root cause (proven via live lag-triage capture + deterministic PTY repro): fish latches feature flags from its startup env before config.fish runs, so the existing no-query-term settings never applied to zellij-spawned shells. With query-term on, fish queries the terminal after every command; one reply zellij fails to relay permanently degrades that fish process's reader. Subshells were immune (inherited the exported var), which is why the lag always 'disappeared' when tested in a new shell. Also adds the lag-triage/unlag/term-probe diagnostic toolkit used to find this, and upstream_repro.py for filing the fish-shell bug.
This commit is contained in:
Noah Masur
@@ -45,6 +45,23 @@ begin
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end >>$logfile 2>&1
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_lt "Captured shell + environment snapshot."
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# Proven root cause of the 2026-08 lag (see docs/CHANGELOG.md 2026-08-29):
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# fish latches feature flags from its startup env before config.fish runs, so
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# a shell with query-term ON sends terminal queries after every command; one
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# reply zellij fails to relay permanently degrades this process's reader.
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if status features | string match -qr '^query-term\s+on'
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_lt ""
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_lt "!! query-term is ON in this shell: fish did NOT get fish_features="
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_lt "!! no-query-term in its STARTUP environment (config.fish is too late)."
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_lt "!! This is the proven root cause of the post-TUI lag — a query reply"
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_lt "!! lost by zellij permanently degrades this fish process's reader."
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_lt "!! Fix: spawn fish with the variable exported (zellij default_shell"
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_lt "!! wrapper fish-no-query-term). Subshells are immune because they"
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_lt "!! inherit the exported variable — that's why a new shell 'fixes' it."
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else
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_lt "query-term is off in this shell (good — the known root cause is ruled out)."
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end
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# ---- 3. Terminal state below the shell --------------------------------------
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_lt ""
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_lt "Querying terminal state (takes a few seconds)..."
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@@ -76,6 +93,10 @@ set -l fixed none
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_lt ""
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_lt "Now applying resets one at a time. After each, type into the test prompt"
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_lt "to judge whether the lag is gone."
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_lt "CAVEAT: fish's read prompt may NOT exhibit lag even when the main"
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_lt "commandline does. If typing at these test prompts never feels laggy at"
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_lt "all, answer 'u' (unsure) instead of 'y' — a 'y' here is only meaningful"
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_lt "if you could feel the lag at the test prompts before the reset."
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if test $fixed = none
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_lt ""
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@@ -0,0 +1,151 @@
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#!/usr/bin/env python3
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"""Deterministic repro of permanent fish reader degradation (fish 4.8.1).
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This is evidence for an upstream fish-shell report, and the proof behind the
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fish-no-query-term wrapper in presets/programs/zellij.nix. Not installed by
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the nix module; run directly: python3 upstream_repro.py [queryterm|noqueryterm]
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Finding: with the query-term feature enabled (latched from the startup env,
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which is fish's default), fish sends OSC 11 + CPR (\e[6n) + DA1 (\e[0c)
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after every external command and waits for the replies. If the terminal
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fails to reply during ONE such cycle -- even though it answered every query
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before and answers every query after -- that fish process's interactive
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reader is PERMANENTLY degraded: keystrokes are no longer echoed (>3s each,
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never recovers). In production this happens when zellij drops/mis-relays a
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reply during TUI teardown or heavy output (cf. zellij-org/zellij#5158), and
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it presents as permanent typing lag cured only by replacing the process.
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With fish_features=no-query-term in the startup environment, the same
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sequence has zero effect (~35ms echo throughout).
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Phases:
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A. terminal answers all queries -> echo ~35ms (both variants)
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B. replies dropped for one command -> queryterm: echo dead, permanently
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C. replies restored, another command -> queryterm: still dead
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"""
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import os, pty, re, select, subprocess, sys, time, fcntl, termios
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VARIANT = sys.argv[1] if len(sys.argv) > 1 else "queryterm"
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env = dict(os.environ)
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env["TERM"] = "xterm-ghostty"
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env["ZELLIJ"] = "0"
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env["ZELLIJ_SESSION_NAME"] = "repro"
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env["FISH_DEBUG"] = "term-support"
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env["FISH_DEBUG_OUTPUT"] = f"/tmp/fish-lagrepro-{VARIANT}.log"
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env.pop("fish_features", None)
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if VARIANT == "noqueryterm":
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env["fish_features"] = "no-query-term"
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master, slave = pty.openpty()
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# give it a size
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fcntl.ioctl(master, termios.TIOCSWINSZ, b"\x00\x28\x00\x78\x00\x00\x00\x00")
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proc = subprocess.Popen(
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["fish", "-i", "--no-config"],
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stdin=slave, stdout=slave, stderr=slave, env=env,
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preexec_fn=lambda: (os.setsid(), fcntl.ioctl(0, termios.TIOCSCTTY, 0)),
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close_fds=True,
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)
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os.close(slave)
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RESPOND = True
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transcript = []
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def respond(data):
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"""Answer terminal queries the way a well-behaved terminal would."""
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out = b""
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for m in re.finditer(rb"\x1b\[6n", data):
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out += b"\x1b[40;1R" # CPR
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for m in re.finditer(rb"\x1b\[0?c", data):
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out += b"\x1b[?62;22c" # DA1
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for m in re.finditer(rb"\x1b\[\?u", data):
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out += b"\x1b[?0u" # kitty flags
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for m in re.finditer(rb"\x1b\]11;\?", data):
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out += b"\x1b]11;rgb:2828/2828/2828\x1b\\" # OSC 11
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for m in re.finditer(rb"\x1b\[>0?q", data):
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out += b"\x1bP>|ghostty 1.3.1\x1b\\" # XTVERSION
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for m in re.finditer(rb"\x1bP\+q[0-9a-fA-F;]+\x1b\\", data):
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out += b"\x1bP0+r\x1b\\" # XTGETTCAP: not found
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for m in re.finditer(rb"\x1b\[\?(\d+)\$p", data):
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out += b"\x1b[?%s;2$y" % m.group(1) # DECRQM: reset
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return out
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def pump(timeout):
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"""Read fish output for `timeout` seconds, answering queries if RESPOND."""
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buf = b""
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end = time.monotonic() + timeout
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while time.monotonic() < end:
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r, _, _ = select.select([master], [], [], 0.03)
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if r:
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try:
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data = os.read(master, 65536)
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except OSError:
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return buf
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buf += data
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transcript.append(data)
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if RESPOND:
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reply = respond(data)
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if reply:
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os.write(master, reply)
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return buf
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def send(s):
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os.write(master, s if isinstance(s, bytes) else s.encode())
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def measure_echo(chars, settle=0.1):
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"""Send chars one at a time; measure time until each is echoed."""
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results = []
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for ch in chars:
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pump(settle)
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t0 = time.monotonic()
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send(ch)
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deadline = time.monotonic() + 3.0
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latency = None
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buf = b""
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while time.monotonic() < deadline:
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buf += pump(0.02)
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if ch.encode() in buf:
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latency = (time.monotonic() - t0) * 1000
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break
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results.append((ch, latency))
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return results
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print(f"=== variant: {VARIANT} ===")
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pump(1.2) # startup, queries answered
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send("echo warmup\r")
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pump(0.8)
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print("phase A: terminal responsive, echo latency per key:")
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for ch, ms in measure_echo("abcde"):
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print(f" {ch}: {ms:.0f} ms" if ms else f" {ch}: NO ECHO in 3s")
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send("\x15") # ctrl-u clear line
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pump(0.3)
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# Run external command, then STOP answering queries (simulate lost relay)
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send("sh -c true\r")
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time.sleep(0.05)
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RESPOND = False
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pump(1.0)
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print("phase B: after external command with query replies DROPPED:")
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for ch, ms in measure_echo("fghij"):
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print(f" {ch}: {ms:.0f} ms" if ms else f" {ch}: NO ECHO in 3s")
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send("\x15")
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pump(0.3)
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# Does it persist across further commands, with responses restored?
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RESPOND = True
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send("sh -c true\r")
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pump(1.0)
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print("phase C: responses restored, after another external command:")
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for ch, ms in measure_echo("klmno"):
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print(f" {ch}: {ms:.0f} ms" if ms else f" {ch}: NO ECHO in 3s")
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send("\x15")
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pump(0.2)
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send("exit\r")
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pump(0.5)
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try:
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proc.wait(timeout=3)
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except subprocess.TimeoutExpired:
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proc.kill()
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@@ -9,6 +9,22 @@ let
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inherit (config.nmasur.settings) username;
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cfg = config.nmasur.presets.programs.zellij;
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# fish latches feature flags from its startup ENVIRONMENT before config.fish
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# runs, so the `set -gx fish_features no-query-term` in config.fish only
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# protects CHILD fish processes — which is why subshells/exec fish were always
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# immune to the post-TUI typing lag while zellij-spawned pane shells were not.
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# With query-term latched on, fish sends OSC 11 + CPR + DA1 queries after
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# every command and waits for replies; if zellij fails to relay even one
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# reply (a race during TUI teardown or heavy output), that fish process's
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# reader is PERMANENTLY degraded — reproduced deterministically in a PTY
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# harness on fish 4.8.1 (see docs/CHANGELOG.md 2026-08-29 and
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# presets/programs/lag-triage/upstream_repro.py). Spawning fish with the
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# variable already exported makes every pane shell immune.
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fish-no-query-term = pkgs.writeShellScriptBin "fish-no-query-term" ''
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export fish_features=no-query-term
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exec ${lib.getExe pkgs.fish} "$@"
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'';
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zellij-switch-to-last = pkgs.writeShellScriptBin "zellij-switch-to-last" ''
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TARGET_SESSION=$(cat ~/.local/state/zellij-last-session)
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if [ -z "$TARGET_SESSION" ]; then
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@@ -130,7 +146,10 @@ in
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# Spawn fish directly instead of trusting $SHELL, which inherits the
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# macOS login shell. On darwin that login shell is no longer managed by
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# nix-darwin, so $SHELL can point at a stale /run/current-system path.
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default_shell = lib.getExe pkgs.fish;
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# Wrapped to export fish_features=no-query-term BEFORE fish starts —
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# see the fish-no-query-term comment above for why this must happen in
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# the environment rather than in config.fish.
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default_shell = lib.getExe fish-no-query-term;
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# default_layout = "compact-top";
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# Remove border
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pane_frames = false;
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