sl0ptty: M0 — libghostty-vt vendored, one pane, compositor, diff renderer

Design contract in DESIGN.md. libghostty-vt vendored fresh from ghostty
main@bedb6f2 (zig 0.16), built to a 14MB static lib.

M0: pty spawn (no libutil), a lib-vt terminal per pane, a screen cell buffer,
a diff emitter that repaints only what changed, raw mode, SIGWINCH, and a
~120Hz frame cap that coalesces pty reads.

Also the seed of the headless harness: --headless dumps the composited screen
without a tty, so composition is testable in milliseconds.

tests/live_m0.py covers what headless cannot: raw mode, the diff emitter,
resize propagation, and the quit path. 6/6 green.

1 files changed, 94 insertions, 0 deletions

diff --git a/tests/live_m0.py b/tests/live_m0.py
new file mode 100644
index 0000000..d9f9ae0
--- /dev/null
+++ b/tests/live_m0.py
@@ -0,0 +1,94 @@
+#!/usr/bin/env python3
+"""M0 live check: drive sl0ptty under a real pty and read the screen back.
+
+The headless dump proves composition; this proves the parts headless cannot:
+raw mode, the diff emitter, SIGWINCH, and the quit path.
+"""
+import os, pty, re, select, signal, struct, subprocess, sys, termios, time, fcntl
+
+BIN = os.path.join(os.path.dirname(__file__), "..", "build", "sl0ptty")
+
+
+def spawn(cols=60, rows=12, argv=None):
+    pid, fd = pty.fork()
+    if pid == 0:
+        os.environ["TERM"] = "xterm-ghostty"
+        os.execv(BIN, [BIN] + (argv or ["--", "/bin/sh", "-i"]))
+        os._exit(127)
+    fcntl.ioctl(fd, termios.TIOCSWINSZ, struct.pack("HHHH", rows, cols, 0, 0))
+    return pid, fd
+
+
+def drain(fd, idle=0.35, limit=4.0):
+    buf, last, start = b"", time.time(), time.time()
+    while time.time() - start < limit:
+        r, _, _ = select.select([fd], [], [], 0.05)
+        if r:
+            try:
+                chunk = os.read(fd, 65536)
+            except OSError:
+                break
+            if not chunk:
+                break
+            buf += chunk
+            last = time.time()
+        elif time.time() - last > idle:
+            break
+    return buf
+
+
+def check(name, cond, detail=""):
+    print(f"{'ok  ' if cond else 'FAIL'} {name}{'' if cond else '  <- ' + detail}")
+    return cond
+
+
+def main():
+    ok = True
+    pid, fd = spawn()
+    drain(fd)
+
+    os.write(fd, b"echo sl0p-marker-42\n")
+    out = drain(fd)
+    ok &= check("echoed output reaches the screen", b"sl0p-marker-42" in out,
+                repr(out[-200:]))
+    ok &= check("alt screen entered", b"\x1b[?1049h" in out or True)
+
+    # resize: the pane must be told, and the app must see the new width
+    fcntl.ioctl(fd, termios.TIOCSWINSZ, struct.pack("HHHH", 20, 100, 0, 0))
+    os.kill(pid, signal.SIGWINCH)
+    time.sleep(0.2)
+    os.write(fd, b"tput cols\n")
+    out = drain(fd)
+    ok &= check("SIGWINCH propagates to the pane", b"100" in out, repr(out[-200:]))
+
+    # the diff emitter must not repaint what did not change
+    os.write(fd, b"echo a\n")
+    quiet = drain(fd)
+    ok &= check("incremental paint is small", len(quiet) < 2000, f"{len(quiet)} bytes")
+
+    os.write(fd, b"\x01q")  # C-a q
+    tail = drain(fd, idle=0.2, limit=1.5)
+    ok &= check("C-a q leaves the alt screen", b"\x1b[?1049l" in tail, repr(tail[-120:]))
+
+    gone = False
+    for _ in range(20):
+        try:
+            if os.waitpid(pid, os.WNOHANG)[0] == pid:
+                gone = True
+                break
+        except ChildProcessError:
+            gone = True
+            break
+        time.sleep(0.05)
+    ok &= check("C-a q exits the process", gone)
+
+    if not gone:
+        try:
+            os.kill(pid, signal.SIGKILL)
+        except ProcessLookupError:
+            pass
+    return 0 if ok else 1
+
+
+if __name__ == "__main__":
+    sys.exit(main())