A run whose output a redraw moved above its start answers cut, never an empty whole

pty/run reads a command's output between its C and D marks. When the
end landed at or before the start, it answered `exit 0` with no output,
as if nothing was printed. That happens when the shell redraws its prompt
over the output, as it can in a window two rows high (winsize 10 2). Now
only equal marks mean nothing was printed. An end at the top-left cell
or above the start says `cut`, since how much was printed is not known.

bash, fish, long lines and 3000-line outputs at winsize 10 2 all came
back whole on this build, so the dogfood's loss could not be reproduced
directly. The test drives the marks into the inverted case the loss
needs.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 7 insertions, 0 deletions

diff --git a/src/ninep/pty.zig b/src/ninep/pty.zig
index 0f5202cc..390ae58d 100644
--- a/src/ninep/pty.zig
+++ b/src/ninep/pty.zig
@@ -960,6 +960,7 @@ test "a run's answer says cut when its output's start is gone, and reads a bound
     var got = sh.answered(p, node, h, into);
     try testing.expect(std.mem.startsWith(u8, got, "exit 0 cut\n"));
 
+
     // A resize between its start and end reflows the pins with the text.
     h = sh.send(p, node);
     sh.put(p, c ++ "one\r\ntwo\r\n");
@@ -1022,6 +1023,12 @@ test "a run's answer says cut when its output's start is gone, and reads a bound
     sh.put(p, d ++ prompt);
     p.gpa = gpa;
     try testing.expectEqualStrings("error out of memory\n", sh.answered(p, node, h, into));
+    // An end drawn above the start (the prompt redrawn over what was
+    // printed, as in a window two rows high): cut, never an empty whole.
+    h = sh.send(p, node);
+    sh.put(p, c ++ "printed\r\nmore\r\n\x1b[4A" ++ d ++ prompt);
+    got = sh.answered(p, node, h, into);
+    try testing.expect(std.mem.startsWith(u8, got, "exit 0 cut\n"));
 }
 
 test "a terminal's ctl counts its body, and a new directory is a rename in the log" {