PTY handling

Lessons extracted from analyzing crynta/terax-ai. Apply them; don't relearn them the hard way.

SIGWINCH on Linux

Linux emits SIGWINCH only when the kernel detects a winsize delta. If you set rows back to the same value, TUI apps (vim, htop, btop) don't repaint.

Pattern: kickPty() bumps rows +1, then restores. Guarantees SIGWINCH. Use it on:

• Tab switch when the previously-released pane was in alt-screen.
• WebGL context restore (after sleep/wake).
• First bind from the renderer pool.

Windows ConPTY race

ConPTY crashes if multiple PTYs spawn concurrently. Acquire a mutex around spawn. Also: ClosePseudoConsole can block indefinitely — do cleanup on a detached thread to prevent IPC stalls.

// pseudocode
let _lock = WINDOWS_CONPTY_LOCK.lock();
let pair = pty_system.openpty(size)?;
drop(_lock);
// ... use pair ...

OSC 7 — current working directory

OSC 7 (\e]7;file://host/path\e\\) reports cwd. But: an attacker could cat a malicious file containing OSC 7 to hijack your shell context.

Trust rule: only honor OSC 7 updates between commands. Track OSC 133 A/B (prompt start) and OSC 133 C/D (command start/end). During C→D (command executing), ignore OSC 7 from the data stream. Update cwd only after D fires.

OSC 133 — prompt markers

Shell integration emits:

• OSC 133 A — prompt start
• OSC 133 B — prompt end / command input start
• OSC 133 C — command executing
• OSC 133 D[;exitcode] — command finished

Use these for: block-style output grouping, AI command palette context, "rerun last command", cwd trust window.

DA filter (Device Attributes)

TUI apps sometimes query \e[c (Device Attributes). If the response is wrong, some apps loop forever.

Pattern: intercept DA queries in a small state machine (3 states: Idle → AfterEsc → InsideCsi), cap buffer at 256 bytes, reply with \e[?1;2c (VT102-compatible). Don't forward DA queries to the frontend.

Coalescing flusher

Reading PTY byte-by-byte and forwarding to the frontend is wasteful (1 IPC call per byte = death). Coalesce:

• Open a 4 ms window from the first byte received.
• Flush when the window expires OR after 50 ms idle (whichever first).
• Cap pending buffer at 4 MiB. Beyond that, drop and emit OVERFLOW_NOTICE (don't OOM on yes | head -c 1GB).

Signal forwarding

• Ctrl+C (in input): send \x03 to the PTY master (the shell handles SIGINT for the foreground process group).
• Ctrl+Z: send \x1a (SIGTSTP). Shell handles backgrounding.
• Ctrl+\: send \x1c (SIGQUIT).
• Window close: send SIGHUP to the PTY foreground process group, then close the master fd.

Do NOT send SIGINT from the parent process directly to the child — pipe through the PTY so the line discipline does the right thing.

Shell integration injection

Inject our prompt markers without touching user dotfiles. Pattern (from Ghostty/Terax):

• zsh: set ZDOTDIR=~/.cache/nyxterm/shell-integration/zsh/, drop our .zshenv/.zshrc there that sources the user's real dotfiles after our hooks.
• bash: launch with --rcfile ~/.cache/nyxterm/shell-integration/bash/bashrc that sources ~/.bashrc last.
• fish: drop a file in ~/.config/fish/conf.d/nyxterm.fish (atomic write + rename).

Atomic file writes always (temp + rename). Never half-write.

Tests to add when this skill is used

• TUI flicker after pane switch (visual + automated).
• High-throughput stream (1 GB/s output) — no OOM, no IPC stall.
• ConPTY concurrent spawn (Windows) — 100 PTYs in parallel.
• OSC 7 trust during cat of crafted file.