An edited row keeps its colours, four copies of forkShell become one, and Esc stops recentring

## A terminal row's ANSI colours survive being edited

The loudest colour bug this editor had: one keystroke anywhere in a coloured shell row turned EVERY
column of it grey. `EditAnchors` anchored a buffer line only when it was BYTE-IDENTICAL to the shell
row it stood over, so a single differing byte dropped the whole row's colour projection. Worst shape
is invisible: append past the pane's right edge, where the text is clipped, and the row looks the
same and only its colour goes.

Anchoring is byte-level now. An edit leaves the row's own bytes at both ends, and being the same
bytes they keep the same colours; only what was typed has no cell under it, so only that takes none.
Live, on real `fastfetch`: a 32-column blue run split into 6 + 26 around one typed character.

Three defects underneath it, all found by machinery rather than by reading:

* A JOIN removes a buffer line while the buffer's covered span grows, so `lines == covered` and both
  aligned guesses — Nth line over the Nth covered row, and the same counted from the bottom —
  resolved to the SAME wrong row. Every untouched row below a join went plain. Anchoring is now a
  streaming monotone matching: one shell-row cursor that only ever moves forward, advanced once per
  buffer line, linear in the buffer where the version before it was quadratic.
* An EMPTY line is not evidence. Splitting a row makes one, it equals every blank row in the span,
  and left free to look ahead it claimed the blank row below the last output and took every coloured
  row in between out of reach of the lines that owned them.
* Reflow under a scrolled viewport. `PageList.getTopLeft(.viewport)` returns the viewport pin
  verbatim, x and all, while `PageList.pin` forces x to 0 — so after a reflow remapped a tracked pin
  into the middle of a row, the text pass dumped row 0 from that column while the colour pass paired
  the fragment with the row's FIRST cells. Row 0 wore its left half's colours until the pane snapped
  back to live output. `bodyText` dumps from column zero now, which is also what ghostty's own
  renderer draws.

Also here: DECSCNM (reverse video) was silently dropped whenever `tty_filter` was off, because the
raw path resolved a `.none` colour by role and never consulted the mode.

The test that found the first two is the one worth keeping: random editing against an ABSOLUTE
oracle — every row's own text names the colour it must have — because the differential oracle it
replaced was blind by construction. It skipped the edited row, which is the row the user is
complaining about.

## Esc returns to a pane without moving its view

Esc in body normal mode runs `Last`, "the pane you were in before this one", and that went through
`focusPaneLine`, which recentred a file on the target line unconditionally. So returning to a buffer
repainted the whole screen to show a line that was already on it.

`focusPaneLine` takes a landing now: `.center` for the three callers going somewhere you have not
been (a look target, a path a pane already holds, `@pN:LINE:COL`), `.keep` for Esc. `.keep` leaves
the view alone and lets `ensureCursorVisible` — which already existed and already scrolls by the
minimum into the `scroll_off` band — be the only thing that may move anything.

Not `line = 0`, which `focusPaneLine` already understands as "focus and touch nothing": a background
pane's view can move while you are away, because the wheel scrolls the pane under the POINTER and a
resize reveals no cursor, so the recorded cursor plus a minimal nudge is what actually gets you back.

Ctrl-o and Ctrl-i keep centring, and the asymmetry is structural rather than arbitrary: `Last` only
ever CROSSES panes, so the pane it lands on already holds the view you left it with, while `jumpBy`
can land in the SAME pane, where a long in-file jump would arrive on the very top or bottom row with
`scroll_off` lines of context on one side. Helix splits the same pair the same way — its jumplist
centres, its buffer switch does not.

One deliberate consequence: under `.keep` a PDF's page is not restored AT ALL, because a page reveal
IS that pane's view and a reveal of the page you are already on still snaps `document_scroll_y` to
that page's start, discarding where you had read to. When something moved the pane while you were
away — the wheel again — Esc leaves it where the wheel left it, and Ctrl-o is how you reach the
recorded page.

## host_io.zig: the machine-local half of a host, once

`host.zig` is the seam. The part of the answer that is identical on every host with an operating
system under it — fork a pane's shell, put bytes on a disk — was written FOUR times: in tty.zig,
gui.zig, macos.zig and detached/server.zig. What those copies had in common says what they were for:
all four were missing FD_CLOEXEC on the pty master, so in every shell pardes has shipped, a program
in one pane could read another pane's terminal.

One copy now, and the wire got smaller for it: `ServerMsg.spawn` is gone. A frontend never asked the
server to fork anything — the server has an operating system under it and forks through `host_io`
like every other host — and `decodeClient` lost the scratch buffer that message needed.

1 files changed, 5 insertions, 52 deletions

diff --git a/src/macos.zig b/src/macos.zig
index e54e013c..2b04237e 100644
--- a/src/macos.zig
+++ b/src/macos.zig
@@ -35,11 +35,8 @@ const file_watch = @import("file_watch.zig");
 /// attachments, so nothing here is analysed.
 const image = if (pardes.pdf_enabled) @import("image.zig") else struct {};
 const user_config = @import("user_config.zig");
+const host_io = @import("host_io.zig");
 
-extern "c" fn forkpty(amaster: *c_int, name: ?[*:0]u8, termp: ?*const anyopaque, winp: ?*const posix.winsize) c_int;
-extern "c" fn execv(path: [*:0]const u8, argv: [*:null]const ?[*:0]const u8) c_int;
-extern "c" fn chdir(path: [*:0]const u8) c_int;
-extern "c" fn _exit(status: c_int) noreturn;
 extern "c" fn setenv(name: [*:0]const u8, value: [*:0]const u8, overwrite: c_int) c_int;
 
 /// Implemented by FileWatcher.swift in the app and e2e host. Zig-only unit
@@ -1807,7 +1804,7 @@ fn spawnShell(ctx: ?*anyopaque, pane: u8, cwd: []const u8) void {
         cwd_buf[cwd.len] = 0;
         cwd_z = @ptrCast(&cwd_buf);
     }
-    const child = forkShell(core, pane, &st.prompt_rcs, core.shellBin(), cwd_z, core.screen_h, core.screen_w);
+    const child = host_io.forkShell(core, pane, &st.prompt_rcs, core.shellBin(), cwd_z, core.screen_h, core.screen_w, null);
     st.ptys[pane] = .{
         .file = child.file,
         .pid = child.pid,
@@ -1823,7 +1820,7 @@ fn spawnShell(ctx: ?*anyopaque, pane: u8, cwd: []const u8) void {
 
 fn ptyWrite(ctx: ?*anyopaque, pane: u8, bytes: []const u8) void {
     const st = hostState(ctx);
-    if (st.ptys[pane]) |pt| writeFd(pt.file.handle, bytes);
+    if (st.ptys[pane]) |pt| host_io.writeFd(pt.file.handle, bytes);
 }
 
 fn ptyResize(ctx: ?*anyopaque, pane: u8, cols: u16, rows: u16) void {
@@ -1848,14 +1845,7 @@ fn ttyTaken(ctx: ?*anyopaque, pane: u8) bool {
 /// resolved which path and which bytes.
 fn writeFile(ctx: ?*anyopaque, pane: u8, path: []const u8, bytes: []const u8) void {
     const st = hostState(ctx);
-    var pathbuf: [4096:0]u8 = undefined;
-    if (path.len >= pathbuf.len) return;
-    @memcpy(pathbuf[0..path.len], path);
-    pathbuf[path.len] = 0;
-    const fd = libc.open(pathbuf[0..path.len :0], .{ .ACCMODE = .WRONLY, .CREAT = true, .TRUNC = true }, @as(libc.mode_t, 0o644));
-    if (fd < 0) return;
-    writeFd(fd, bytes);
-    _ = libc.close(fd);
+    if (!host_io.writeFileBytes(path, bytes)) return;
     // The directory source will observe our own close. Move its baseline first
     // so that notification is a hash no-op instead of manufacturing an external
     // reload and undo boundary.
@@ -1870,10 +1860,7 @@ fn writeDump(ctx: ?*anyopaque, bytes: []const u8) void {
     const st = hostState(ctx);
     var pbuf: [1024:0]u8 = undefined;
     const path = pardes.dump.outPath(&pbuf) orelse return;
-    const fd = libc.open(path, .{ .ACCMODE = .WRONLY, .CREAT = true, .TRUNC = true }, @as(libc.mode_t, 0o644));
-    if (fd < 0) return;
-    writeFd(fd, bytes);
-    _ = libc.close(fd);
+    if (!host_io.writeFileBytes(path, bytes)) return;
     st.core.setLastDump(path);
 }
 
@@ -1988,40 +1975,6 @@ fn wake(st: *State) void {
 
 // ---------------------------------------------------------------- helpers
 
-fn forkShell(core: *pardes.Pardes, pane: usize, prompt_rcs: *const shell_bin.PromptRcs, bin: []const u8, cwd: ?[*:0]const u8, rows: u16, cols: u16) struct { file: std.Io.File, pid: posix.pid_t } {
-    var master: c_int = undefined;
-    // Resolved BEFORE the fork, into this frame, which the child inherits:
-    // nothing between fork and exec may allocate, so a PATH search cannot
-    // happen there.
-    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
-    const spawn = shell_bin.resolve(bin, &path_buf, prompt_rcs);
-    const ws = posix.winsize{ .row = rows, .col = cols, .xpixel = 0, .ypixel = 0 };
-    const pid = forkpty(&master, null, null, &ws);
-    if (pid == 0) {
-        if (cwd) |c| _ = chdir(c);
-        _ = execv(spawn.path, &spawn.argv);
-        _exit(127);
-    }
-    if (pid > 0) core.acknowledgeShell(pane, std.mem.span(spawn.path), spawn.argv[1] != null);
-    return .{ .file = .{ .handle = master, .flags = .{ .nonblocking = false } }, .pid = pid };
-}
-
-fn writeFd(fd: c_int, data: []const u8) void {
-    var off: usize = 0;
-    while (off < data.len) {
-        const n = libc.write(fd, data[off..].ptr, data.len - off);
-        if (n < 0) {
-            if (libc.errno(n) == .INTR) continue;
-            return;
-        }
-        // A zero-byte write makes no progress; looping on it would spin the
-        // main thread forever, which here means a beachball rather than the
-        // tty shell's hung terminal.
-        if (n == 0) return;
-        off += @intCast(n);
-    }
-}
-
 /// Rebuild the process environment as a Map, because a library never sees the
 /// std.process.Init that main() gets one from. Only the config-path lookup
 /// reads it, and the arena owns the copies for the life of the process.