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.

docs/CHANGELOG.md

@@ -1,5 +1,23 @@
 # Changelog
 
+## 2026-08-29 (root cause found and fixed)
+
+- **Root-caused and fixed the recurring post-TUI typing lag** (fish + Zellij + Ghostty) using a `lag-triage` capture from a live lagging shell plus a deterministic PTY reproduction (`presets/programs/lag-triage/upstream_repro.py`):
+  - **Root cause chain**: (1) fish latches feature flags from its **startup environment**, before `config.fish` runs — so the existing `set -gx fish_features no-query-term` in `shellInit` never applied to the shell that set it, only to its children. (2) Zellij spawns pane shells via `default_shell` with no `fish_features` in the environment, so every pane's fish latched `query-term` **on** (the fish 4.8.1 default; the triage log from the lagging shell confirmed `query-term on` while `$fish_features` was correctly set to `no-query-term`). (3) With query-term on, fish sends OSC 11 + CPR (`\e[6n`) + DA1 (`\e[0c`) after **every** command and waits for replies relayed by Zellij. (4) Reproduced on fish 4.8.1: if the terminal fails to reply during just **one** such cycle — answering everything before and after — that fish process's interactive reader is **permanently degraded** (keystroke echo >3s, never recovers; ~35ms before). In production Zellij drops/mangles a relay during TUI teardown or heavy output (cf. zellij-org/zellij#5158), e.g. after `nh home switch`, nvim, jjui, yazi.
+  - **Why every previous observation finally makes sense**: subshells and `exec fish` were never "resetting" anything — they *inherited* the exported `fish_features=no-query-term` from config.fish, latched query-term off at startup, and were therefore **immune**. The parent zellij-spawned shell never had the variable at startup and stayed vulnerable. Raw keystroke capture in the lagging pane showed instant plain bytes (input path fine) and no stuck terminal modes — the damage was inside the fish process, exactly as the repro shows.
+  - **Fix**: `zellij.nix` now spawns panes through a `fish-no-query-term` wrapper (`export fish_features=no-query-term; exec fish`), so the feature is latched off in every pane shell. Verified: interactive fish through the built wrapper with the real config reports `query-term off`; the PTY repro with `no-query-term` in the environment shows ~35ms echo through all failure phases.
+  - **Correction** to the earlier 2026-08-29 entry: `query-term` does **not** default to off in fish 4.8.1 — it defaults on; it only *appeared* off in non-interactive checks because the user config's `set -gx` takes effect for `fish -c` (no reader latch) but not for interactive shells.
+  - Upstream: fish-shell should bound the reader's wait for query replies instead of degrading permanently (repro script kept at `presets/programs/lag-triage/upstream_repro.py` for filing); Zellij's reply relaying is the trigger (zellij-org/zellij#5158).
+  - `lag-triage` now checks `status features` and calls out `query-term on` as the known root cause, and warns that its `read`-prompt typing tests may not exhibit main-commandline lag (which produced a false "fixed by stage A" in the first capture).
+
+## 2026-08-29 (later)
+
+- Added a diagnostic toolkit (`lag-triage` / `unlag` fish functions + `term-probe` binary, `presets/programs/lag-triage/`) for the still-recurring post-TUI typing lag in fish + Zellij + Ghostty, instead of another blind fix. Findings that motivated it:
+  - All three prior fixes were either no-ops or insufficient: `fish_features = no-query-term` is a **no-op** because `query-term` already defaults to *off* in fish 4.8.1 (verified with `status features`); disabling Ghostty's fish integration inside Zellij and setting `support_kitty_keyboard_protocol = false` did not stop recurrence.
+  - PTY captures of fish 4.8.1 (`TERM=xterm-256color`, with and without `$ZELLIJ`) show fish never writes Kitty keyboard sequences to the wire — it uses modifyOtherKeys (`\e[>4;1m`), application keypad (`\e=`), bracketed paste (`?2004`), and color-theme reporting (`?2031`), enabling them at every prompt and disabling them before every external command. Crucially, a fresh subshell's startup bytes are identical to the parent's post-command re-enable bytes, so "a subshell fixes the lag" cannot be explained by a simple terminal-state reset — leaving two competing hypotheses that only live capture can separate: (1) fish-internal reader state poisoned by stray/partial escape bytes (e.g. leaked from a closing floating pane), cleared only by a new fish process; (2) Zellij/Ghostty-level stuck state (Zellij 0.45's `StdinAnsiParser` is already a proven source of input delays — see the Alt-Shift-P fix below).
+  - Also note: the floating-pane TUIs (jjui via Alt-Shift-J, yazi via Alt-Shift-Y, scrollback editor) run in their own panes and never pass through the shell's fish process at all, while `nvim` runs inside the shell pane — the triage log records which path preceded the lag.
+  - **Next occurrence: run `lag-triage` in the lagging shell BEFORE starting a new shell.** It snapshots the environment, queries pane terminal state (kitty flags, modifyOtherKeys, DEC modes, DA1 round-trip latency), captures raw keystroke bytes+timing bypassing fish, then applies staged resets (kitty pop/clear, modifyOtherKeys off, keypad/cursor, mouse/focus/sync, altscreen, stty, DECSTR) — the stage that cures it names the stuck layer. Logs to `~/.local/state/lag-triage/` for an upstream issue. `unlag` is the one-shot convenience version (if `unlag` never helps but `exec fish` does, the bug is fish-internal).
+
 ## 2026-08-29
 
 - Fixed 1.5-second latency when pressing `Alt-Shift-P` to trigger `zellij-session` in Zellij 0.45.0 + Ghostty:

platforms/home-manager/modules/nmasur/presets/programs/lag-triage/default.nix

@@ -0,0 +1,38 @@
+{
+  config,
+  pkgs,
+  lib,
+  ...
+}:
+
+let
+  cfg = config.nmasur.presets.programs.lag-triage;
+
+  term-probe = pkgs.writeScriptBin "term-probe" ''
+    #!${lib.getExe pkgs.python3}
+    ${builtins.readFile ./term_probe.py}
+  '';
+in
+
+{
+
+  options.nmasur.presets.programs.lag-triage.enable =
+    lib.mkEnableOption "Terminal input-lag triage tools";
+
+  config = lib.mkIf cfg.enable {
+
+    home.packages = [ term-probe ];
+
+    programs.fish.functions = {
+      lag-triage = {
+        description = "Diagnose post-TUI typing lag in the current shell";
+        body = builtins.readFile ./lag-triage.fish;
+      };
+      unlag = {
+        description = "Reset terminal state left behind by a TUI";
+        body = builtins.readFile ./unlag.fish;
+      };
+    };
+
+  };
+}

platforms/home-manager/modules/nmasur/presets/programs/lag-triage/lag-triage.fish

@@ -0,0 +1,182 @@
+# Guided diagnosis for the post-TUI typing-lag problem (Ghostty + Zellij + fish).
+# Run this IN THE LAGGING SHELL the moment you notice the lag, BEFORE starting
+# a new shell. It captures evidence, then applies targeted resets one at a time
+# so the stage that cures the lag identifies the layer holding stuck state.
+# Everything is logged for filing an upstream issue.
+
+set -l logdir ~/.local/state/lag-triage
+mkdir -p $logdir
+set -l logfile $logdir/(date +%Y%m%d-%H%M%S).log
+
+function _lt --inherit-variable logfile
+    echo $argv | tee -a $logfile
+end
+
+function _lt_ask --inherit-variable logfile
+    # usage: _lt_ask VARNAME prompt... -> sets global $VARNAME (default: skip)
+    set -l __name $argv[1]
+    read -g -P "$argv[2..] " $__name
+    or set -g $__name skip
+    test -z "$$__name"; and set -g $__name skip
+    echo "ANSWER $__name: $$__name" >>$logfile
+end
+
+_lt "== lag-triage "(date)" =="
+_lt "Log: $logfile"
+_lt "Answer y / n, or press Enter to skip a question."
+_lt ""
+
+# ---- 1. Context -------------------------------------------------------------
+_lt_ask ans_tui "Which TUI did you just exit (nvim/jjui/yazi/other)?"
+_lt_ask ans_launch "Launched via (f)loating-pane keybind or (c)ommand typed in this shell?"
+
+# ---- 2. Snapshot ------------------------------------------------------------
+begin
+    echo "-- snapshot --"
+    fish --version
+    echo "fish pid: $fish_pid, started: "(ps -o lstart= -p $fish_pid 2>/dev/null)
+    zellij --version 2>/dev/null
+    echo "escape delay: '$fish_escape_delay_ms' sequence delay: '$fish_sequence_key_delay_ms'"
+    env | grep -iE '^(TERM|ZELLIJ|GHOSTTY|COLORTERM)' | sort
+    echo "-- status features --"
+    status features
+    echo "-- stty -a --"
+    stty -a
+end >>$logfile 2>&1
+_lt "Captured shell + environment snapshot."
+
+# Proven root cause of the 2026-08 lag (see docs/CHANGELOG.md 2026-08-29):
+# fish latches feature flags from its startup env before config.fish runs, so
+# a shell with query-term ON sends terminal queries after every command; one
+# reply zellij fails to relay permanently degrades this process's reader.
+if status features | string match -qr '^query-term\s+on'
+    _lt ""
+    _lt "!! query-term is ON in this shell: fish did NOT get fish_features="
+    _lt "!! no-query-term in its STARTUP environment (config.fish is too late)."
+    _lt "!! This is the proven root cause of the post-TUI lag — a query reply"
+    _lt "!! lost by zellij permanently degrades this fish process's reader."
+    _lt "!! Fix: spawn fish with the variable exported (zellij default_shell"
+    _lt "!! wrapper fish-no-query-term). Subshells are immune because they"
+    _lt "!! inherit the exported variable — that's why a new shell 'fixes' it."
+else
+    _lt "query-term is off in this shell (good — the known root cause is ruled out)."
+end
+
+# ---- 3. Terminal state below the shell --------------------------------------
+_lt ""
+_lt "Querying terminal state (takes a few seconds)..."
+term-probe report 2>&1 | tee -a $logfile
+_lt ""
+_lt "  ^ Things to look for: kitty flags with a reply > 0, modifyOtherKeys > 1,"
+_lt "    any mouse/alternate-screen mode SET while at a shell prompt, or a slow"
+_lt "    DA1 round-trip (> 100 ms means the input path itself is delayed)."
+
+# ---- 4. Raw keystroke capture (bypasses fish entirely) ----------------------
+_lt ""
+_lt "Raw input capture: type ~10 characters at a steady pace, including one"
+_lt "ESC press and one arrow key. This shows the exact bytes this pane delivers"
+_lt "and their timing, with fish's input handling out of the picture."
+term-probe keylog 2>&1 | tee -a $logfile
+_lt_ask ans_keylog_instant "Did each keypress appear INSTANTLY in the capture? (y/n)"
+_lt_ask ans_keylog_plain "Were plain letters single plain bytes like b'a' (not escape sequences)? (y/n)"
+
+# ---- 5. Scope ---------------------------------------------------------------
+_lt ""
+_lt_ask ans_scope_pane "Optional: open a NEW zellij pane/tab and type — laggy there too? (y/n)"
+_lt_ask ans_scope_window "Optional: type in a separate Ghostty window (outside this zellij session) — laggy? (y/n)"
+
+# ---- 6. Staged resets -------------------------------------------------------
+# Each stage resets one category of state a TUI could have left behind.
+# The first stage that cures the lag names the culprit.
+set -l fixed none
+
+_lt ""
+_lt "Now applying resets one at a time. After each, type into the test prompt"
+_lt "to judge whether the lag is gone."
+_lt "CAVEAT: fish's read prompt may NOT exhibit lag even when the main"
+_lt "commandline does. If typing at these test prompts never feels laggy at"
+_lt "all, answer 'u' (unsure) instead of 'y' — a 'y' here is only meaningful"
+_lt "if you could feel the lag at the test prompts before the reset."
+
+if test $fixed = none
+    _lt ""
+    _lt "Stage A - kitty keyboard protocol: pop stack + clear all flags"
+    printf '\e[<9u\e[=0;1u'
+    _lt_ask ans_stage_a "  Test typing here, then Enter — lag gone? (y/n)"
+    test "$ans_stage_a" = y; and set fixed "A (kitty keyboard state)"
+end
+
+if test $fixed = none
+    _lt "Stage B - modifyOtherKeys off"
+    printf '\e[>4;0m'
+    _lt_ask ans_stage_b "  Test typing here, then Enter — lag gone? (y/n)"
+    test "$ans_stage_b" = y; and set fixed "B (modifyOtherKeys)"
+end
+
+if test $fixed = none
+    _lt "Stage C - normal keypad + normal cursor keys"
+    printf '\e>\e[?1l'
+    _lt_ask ans_stage_c "  Test typing here, then Enter — lag gone? (y/n)"
+    test "$ans_stage_c" = y; and set fixed "C (application keypad/cursor mode)"
+end
+
+if test $fixed = none
+    _lt "Stage D - disable mouse, focus reporting, synchronized output"
+    printf '\e[?1000l\e[?1001l\e[?1002l\e[?1003l\e[?1005l\e[?1006l\e[?1015l\e[?1016l\e[?1004l\e[?2026l'
+    _lt_ask ans_stage_d "  Test typing here, then Enter — lag gone? (y/n)"
+    test "$ans_stage_d" = y; and set fixed "D (mouse/focus/sync modes)"
+end
+
+if test $fixed = none
+    _lt "Stage E - leave alternate screen"
+    printf '\e[?1049l'
+    _lt_ask ans_stage_e "  Test typing here, then Enter — lag gone? (y/n)"
+    test "$ans_stage_e" = y; and set fixed "E (alternate screen)"
+end
+
+if test $fixed = none
+    _lt "Stage F - stty sane (line-discipline reset)"
+    stty sane
+    _lt_ask ans_stage_f "  Test typing here, then Enter — lag gone? (y/n)"
+    test "$ans_stage_f" = y; and set fixed "F (termios/line discipline)"
+end
+
+if test $fixed = none
+    _lt "Stage G - DECSTR soft terminal reset"
+    printf '\e[!p'
+    _lt_ask ans_stage_g "  Test typing here, then Enter — lag gone? (y/n)"
+    test "$ans_stage_g" = y; and set fixed "G (DECSTR-resettable mode)"
+end
+
+# ---- 7. Verdict --------------------------------------------------------------
+_lt ""
+_lt "== Verdict =="
+if test $fixed != none
+    _lt "Lag cleared by stage $fixed."
+    _lt "That state was stuck BELOW fish — in the Zellij pane or relayed to"
+    _lt "Ghostty — and the TUI you exited ($ans_tui) failed to restore it, or"
+    _lt "Zellij failed to restore it when the pane closed."
+    _lt "Re-probing terminal state after the fix for comparison:"
+    term-probe report 2>&1 | tee -a $logfile
+    _lt ""
+    _lt "-> File this log against zellij (or ghostty, if a separate window also"
+    _lt "   lagged). The before/after probe diff pinpoints the exact stuck mode."
+else if test "$ans_keylog_instant" = y; and test "$ans_keylog_plain" = y
+    _lt "Raw input reaches this pane instantly as plain bytes, and no terminal"
+    _lt "state reset helps: the lag lives INSIDE this fish process (reader state)."
+    _lt "Confirm now: run 'exec fish' — if that cures it, it is fish-internal."
+    _lt ""
+    _lt "-> To catch it in the act, run your next long-lived shell as:"
+    _lt "   FISH_DEBUG='reader,term-support' FISH_DEBUG_OUTPUT=$logdir/fish-debug.log fish"
+    _lt "   then re-run lag-triage when it recurs and file both logs to fish-shell."
+else
+    _lt "Keystrokes were delayed or arrived as escape sequences BEFORE fish saw"
+    _lt "them: the problem is in Zellij (client stdin parser / server) or Ghostty."
+    _lt "  new pane also laggy:        $ans_scope_pane  (y -> session-wide, not this pane)"
+    _lt "  separate window laggy:      $ans_scope_window  (y -> Ghostty itself)"
+    _lt "-> File this log against zellij; include the keylog byte capture."
+end
+_lt ""
+_lt "Full log: $logfile"
+
+functions -e _lt _lt_ask

platforms/home-manager/modules/nmasur/presets/programs/lag-triage/term_probe.py

@@ -0,0 +1,165 @@
+"""Probe the terminal state of the current pane, below the shell.
+
+Modes:
+  report  - query kitty-keyboard flags, modifyOtherKeys, and DEC private
+            modes directly on /dev/tty, reporting each reply (or lack of
+            one) and its round-trip latency. Answers may come from Zellij
+            (pane state) or be relayed from Ghostty (window state).
+  keylog  - raw-mode keystroke capture: prints the exact bytes and
+            inter-key latency for every keypress, bypassing the shell's
+            input machinery entirely. Press q to finish.
+
+Used by the `lag-triage` fish function to pin down which layer
+(fish / zellij / ghostty) is holding stuck state when typing lags
+after a TUI exits.
+"""
+
+import os
+import re
+import select
+import sys
+import termios
+import time
+import tty
+
+# DECRQM reply values
+DECRQM_VALUES = {
+    "0": "not recognized",
+    "1": "SET",
+    "2": "reset",
+    "3": "permanently set",
+    "4": "permanently reset",
+}
+
+DEC_MODES = [
+    (1, "application cursor keys (DECCKM)"),
+    (25, "cursor visible"),
+    (1000, "mouse click reporting"),
+    (1002, "mouse drag reporting"),
+    (1003, "mouse all-motion reporting"),
+    (1004, "focus reporting"),
+    (1006, "SGR mouse encoding"),
+    (1049, "alternate screen"),
+    (2004, "bracketed paste"),
+    (2026, "synchronized output"),
+    (2031, "color theme reporting"),
+]
+
+QUERIES = [
+    ("kitty keyboard flags (\\e[?u)", b"\x1b[?u", rb"\x1b\[\?(\d+)u", None),
+    ("modifyOtherKeys (XTQMODKEYS)", b"\x1b[?4m", rb"\x1b\[>4;(\d+)m", None),
+    ("background color (OSC 11)", b"\x1b]11;?\x1b\\", rb"\x1b\]11;([^\x07\x1b]+)", None),
+] + [
+    (
+        f"DEC mode {num} — {desc}",
+        b"\x1b[?%d$p" % num,
+        rb"\x1b\[\?%d;(\d+)\$y" % num,
+        DECRQM_VALUES,
+    )
+    for num, desc in DEC_MODES
+]
+
+
+def read_for(fd, seconds):
+    buf = b""
+    end = time.monotonic() + seconds
+    while True:
+        remaining = end - time.monotonic()
+        if remaining <= 0:
+            break
+        r, _, _ = select.select([fd], [], [], remaining)
+        if not r:
+            break
+        buf += os.read(fd, 4096)
+    return buf
+
+
+def report(fd):
+    lines = []
+    raw_dump = b""
+    for label, query, pattern, value_names in QUERIES:
+        raw_dump += read_for(fd, 0.02)  # drain stragglers
+        start = time.monotonic()
+        os.write(fd, query)
+        buf = b""
+        match = None
+        deadline = time.monotonic() + 0.35
+        while time.monotonic() < deadline:
+            buf += read_for(fd, 0.05)
+            match = re.search(pattern, buf)
+            if match:
+                break
+        raw_dump += buf
+        if match:
+            latency = (time.monotonic() - start) * 1000
+            value = match.group(1).decode("ascii", "replace")
+            if value_names:
+                value = f"{value} ({value_names.get(value, '?')})"
+            lines.append(f"  {label:45s} = {value:24s} [{latency:6.1f} ms]")
+        else:
+            lines.append(f"  {label:45s} = (no reply)")
+
+    # DA1 as a fence: every terminal answers it, so its round-trip time
+    # measures the whole input path (ghostty -> zellij -> pane -> here).
+    start = time.monotonic()
+    os.write(fd, b"\x1b[c")
+    buf = b""
+    match = None
+    deadline = time.monotonic() + 2.0
+    while time.monotonic() < deadline:
+        buf += read_for(fd, 0.05)
+        match = re.search(rb"\x1b\[\?([0-9;]*)c", buf)
+        if match:
+            break
+    raw_dump += buf
+    if match:
+        latency = (time.monotonic() - start) * 1000
+        lines.append(
+            f"  {'device attributes (DA1) round-trip':45s} = "
+            f"{match.group(1).decode():24s} [{latency:6.1f} ms]"
+        )
+    else:
+        lines.append(f"  {'device attributes (DA1) round-trip':45s} = (NO REPLY in 2s!)")
+    lines.append(f"  raw bytes received: {raw_dump!r}")
+    return lines
+
+
+def keylog(fd):
+    sys.stdout.write("keylog: capturing raw bytes from the tty. Press q to finish.\r\n")
+    sys.stdout.flush()
+    last = time.monotonic()
+    while True:
+        select.select([fd], [], [], None)
+        data = os.read(fd, 4096)
+        now = time.monotonic()
+        delta_ms = (now - last) * 1000
+        last = now
+        sys.stdout.write(f"  +{delta_ms:8.1f} ms  {data!r}  hex={data.hex(' ')}\r\n")
+        sys.stdout.flush()
+        if data in (b"q", b"\x03", b"\x04"):
+            break
+
+
+def main():
+    mode = sys.argv[1] if len(sys.argv) > 1 else "report"
+    fd = os.open("/dev/tty", os.O_RDWR)
+    old = termios.tcgetattr(fd)
+    lines = None
+    try:
+        tty.setraw(fd)
+        if mode == "report":
+            lines = report(fd)
+        elif mode == "keylog":
+            keylog(fd)
+        else:
+            raise SystemExit(f"unknown mode: {mode}")
+    finally:
+        termios.tcsetattr(fd, termios.TCSADRAIN, old)
+        os.close(fd)
+    if lines:
+        print("terminal state as seen from this pane:")
+        print("\n".join(lines))
+
+
+if __name__ == "__main__":
+    main()

platforms/home-manager/modules/nmasur/presets/programs/lag-triage/unlag.fish

@@ -0,0 +1,15 @@
+# One-shot reset of terminal state a TUI may have left behind (kitty keyboard
+# flags, modifyOtherKeys, application keypad/cursor, mouse/focus reporting,
+# alternate screen, termios). fish re-enables the modes it wants at the next
+# prompt, so this is safe to run any time.
+#
+# Diagnostic value: if this cures the lag, the stuck state was below fish
+# (run lag-triage next time to find which mode). If only `exec fish` cures
+# it, the lag is inside the fish process itself.
+printf '\e[<9u\e[=0;1u'
+printf '\e[>4;0m'
+printf '\e>\e[?1l'
+printf '\e[?1000l\e[?1001l\e[?1002l\e[?1003l\e[?1005l\e[?1006l\e[?1015l\e[?1016l\e[?1004l\e[?2026l'
+printf '\e[?1049l'
+stty sane
+echo "terminal state reset — if typing still lags, run lag-triage (before exec fish!)"

platforms/home-manager/modules/nmasur/presets/programs/lag-triage/upstream_repro.py

@@ -0,0 +1,151 @@
+#!/usr/bin/env python3
+"""Deterministic repro of permanent fish reader degradation (fish 4.8.1).
+
+This is evidence for an upstream fish-shell report, and the proof behind the
+fish-no-query-term wrapper in presets/programs/zellij.nix. Not installed by
+the nix module; run directly: python3 upstream_repro.py [queryterm|noqueryterm]
+
+Finding: with the query-term feature enabled (latched from the startup env,
+which is fish's default), fish sends OSC 11 + CPR (\e[6n) + DA1 (\e[0c)
+after every external command and waits for the replies. If the terminal
+fails to reply during ONE such cycle -- even though it answered every query
+before and answers every query after -- that fish process's interactive
+reader is PERMANENTLY degraded: keystrokes are no longer echoed (>3s each,
+never recovers). In production this happens when zellij drops/mis-relays a
+reply during TUI teardown or heavy output (cf. zellij-org/zellij#5158), and
+it presents as permanent typing lag cured only by replacing the process.
+With fish_features=no-query-term in the startup environment, the same
+sequence has zero effect (~35ms echo throughout).
+
+Phases:
+  A. terminal answers all queries        -> echo ~35ms (both variants)
+  B. replies dropped for one command     -> queryterm: echo dead, permanently
+  C. replies restored, another command   -> queryterm: still dead
+"""
+import os, pty, re, select, subprocess, sys, time, fcntl, termios
+
+VARIANT = sys.argv[1] if len(sys.argv) > 1 else "queryterm"
+
+env = dict(os.environ)
+env["TERM"] = "xterm-ghostty"
+env["ZELLIJ"] = "0"
+env["ZELLIJ_SESSION_NAME"] = "repro"
+env["FISH_DEBUG"] = "term-support"
+env["FISH_DEBUG_OUTPUT"] = f"/tmp/fish-lagrepro-{VARIANT}.log"
+env.pop("fish_features", None)
+if VARIANT == "noqueryterm":
+    env["fish_features"] = "no-query-term"
+
+master, slave = pty.openpty()
+# give it a size
+fcntl.ioctl(master, termios.TIOCSWINSZ, b"\x00\x28\x00\x78\x00\x00\x00\x00")
+proc = subprocess.Popen(
+    ["fish", "-i", "--no-config"],
+    stdin=slave, stdout=slave, stderr=slave, env=env,
+    preexec_fn=lambda: (os.setsid(), fcntl.ioctl(0, termios.TIOCSCTTY, 0)),
+    close_fds=True,
+)
+os.close(slave)
+
+RESPOND = True
+transcript = []
+
+def respond(data):
+    """Answer terminal queries the way a well-behaved terminal would."""
+    out = b""
+    for m in re.finditer(rb"\x1b\[6n", data):
+        out += b"\x1b[40;1R"                       # CPR
+    for m in re.finditer(rb"\x1b\[0?c", data):
+        out += b"\x1b[?62;22c"                     # DA1
+    for m in re.finditer(rb"\x1b\[\?u", data):
+        out += b"\x1b[?0u"                         # kitty flags
+    for m in re.finditer(rb"\x1b\]11;\?", data):
+        out += b"\x1b]11;rgb:2828/2828/2828\x1b\\" # OSC 11
+    for m in re.finditer(rb"\x1b\[>0?q", data):
+        out += b"\x1bP>|ghostty 1.3.1\x1b\\"       # XTVERSION
+    for m in re.finditer(rb"\x1bP\+q[0-9a-fA-F;]+\x1b\\", data):
+        out += b"\x1bP0+r\x1b\\"                   # XTGETTCAP: not found
+    for m in re.finditer(rb"\x1b\[\?(\d+)\$p", data):
+        out += b"\x1b[?%s;2$y" % m.group(1)        # DECRQM: reset
+    return out
+
+def pump(timeout):
+    """Read fish output for `timeout` seconds, answering queries if RESPOND."""
+    buf = b""
+    end = time.monotonic() + timeout
+    while time.monotonic() < end:
+        r, _, _ = select.select([master], [], [], 0.03)
+        if r:
+            try:
+                data = os.read(master, 65536)
+            except OSError:
+                return buf
+            buf += data
+            transcript.append(data)
+            if RESPOND:
+                reply = respond(data)
+                if reply:
+                    os.write(master, reply)
+    return buf
+
+def send(s):
+    os.write(master, s if isinstance(s, bytes) else s.encode())
+
+def measure_echo(chars, settle=0.1):
+    """Send chars one at a time; measure time until each is echoed."""
+    results = []
+    for ch in chars:
+        pump(settle)
+        t0 = time.monotonic()
+        send(ch)
+        deadline = time.monotonic() + 3.0
+        latency = None
+        buf = b""
+        while time.monotonic() < deadline:
+            buf += pump(0.02)
+            if ch.encode() in buf:
+                latency = (time.monotonic() - t0) * 1000
+                break
+        results.append((ch, latency))
+    return results
+
+print(f"=== variant: {VARIANT} ===")
+pump(1.2)  # startup, queries answered
+
+send("echo warmup\r")
+pump(0.8)
+
+print("phase A: terminal responsive, echo latency per key:")
+for ch, ms in measure_echo("abcde"):
+    print(f"  {ch}: {ms:.0f} ms" if ms else f"  {ch}: NO ECHO in 3s")
+send("\x15")  # ctrl-u clear line
+pump(0.3)
+
+# Run external command, then STOP answering queries (simulate lost relay)
+send("sh -c true\r")
+time.sleep(0.05)
+RESPOND = False
+pump(1.0)
+
+print("phase B: after external command with query replies DROPPED:")
+for ch, ms in measure_echo("fghij"):
+    print(f"  {ch}: {ms:.0f} ms" if ms else f"  {ch}: NO ECHO in 3s")
+send("\x15")
+pump(0.3)
+
+# Does it persist across further commands, with responses restored?
+RESPOND = True
+send("sh -c true\r")
+pump(1.0)
+print("phase C: responses restored, after another external command:")
+for ch, ms in measure_echo("klmno"):
+    print(f"  {ch}: {ms:.0f} ms" if ms else f"  {ch}: NO ECHO in 3s")
+
+send("\x15")
+pump(0.2)
+send("exit\r")
+pump(0.5)
+try:
+    proc.wait(timeout=3)
+except subprocess.TimeoutExpired:
+    proc.kill()

platforms/home-manager/modules/nmasur/presets/programs/zellij.nix

@@ -9,6 +9,22 @@ let
   inherit (config.nmasur.settings) username;
   cfg = config.nmasur.presets.programs.zellij;
 
+  # fish latches feature flags from its startup ENVIRONMENT before config.fish
+  # runs, so the `set -gx fish_features no-query-term` in config.fish only
+  # protects CHILD fish processes — which is why subshells/exec fish were always
+  # immune to the post-TUI typing lag while zellij-spawned pane shells were not.
+  # With query-term latched on, fish sends OSC 11 + CPR + DA1 queries after
+  # every command and waits for replies; if zellij fails to relay even one
+  # reply (a race during TUI teardown or heavy output), that fish process's
+  # reader is PERMANENTLY degraded — reproduced deterministically in a PTY
+  # harness on fish 4.8.1 (see docs/CHANGELOG.md 2026-08-29 and
+  # presets/programs/lag-triage/upstream_repro.py). Spawning fish with the
+  # variable already exported makes every pane shell immune.
+  fish-no-query-term = pkgs.writeShellScriptBin "fish-no-query-term" ''
+    export fish_features=no-query-term
+    exec ${lib.getExe pkgs.fish} "$@"
+  '';
+
   zellij-switch-to-last = pkgs.writeShellScriptBin "zellij-switch-to-last" ''
     TARGET_SESSION=$(cat ~/.local/state/zellij-last-session)
     if [ -z "$TARGET_SESSION" ]; then
@@ -130,7 +146,10 @@ in
         # Spawn fish directly instead of trusting $SHELL, which inherits the
         # macOS login shell. On darwin that login shell is no longer managed by
         # nix-darwin, so $SHELL can point at a stale /run/current-system path.
-        default_shell = lib.getExe pkgs.fish;
+        # Wrapped to export fish_features=no-query-term BEFORE fish starts —
+        # see the fish-no-query-term comment above for why this must happen in
+        # the environment rather than in config.fish.
+        default_shell = lib.getExe fish-no-query-term;
         # default_layout = "compact-top";
         # Remove border
         pane_frames = false;

platforms/home-manager/modules/nmasur/profiles/power-user.nix

@@ -63,6 +63,7 @@ in
       git.enable = lib.mkDefault true;
       helix.enable = lib.mkDefault true;
       jujutsu.enable = lib.mkDefault true;
+      lag-triage.enable = lib.mkDefault true;
       lazygit.enable = lib.mkDefault true;
       # neovim.enable = lib.mkDefault true;
       nix-index.enable = lib.mkDefault true;