pty/run after a clear answers what the command printed from the clear on, whole; cut only when its start really scrolled away

A clear (ED 2, ED 3, a reset) under a running command marked its output
lost, so `clear; ls` answered only "exit 0 cut". The output's start is
now pinned again at the screen's top after the erase: what the command
prints from there is its answer, whole, and a cut is said only when
that start leaves the scrollback.

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

1 files changed, 3 insertions, 4 deletions

diff --git a/src/ninep/pty.zig b/src/ninep/pty.zig
index 2245255b..fc8eb570 100644
--- a/src/ninep/pty.zig
+++ b/src/ninep/pty.zig
@@ -971,13 +971,12 @@ test "a run's answer says cut when its output's start is gone, and reads a bound
     defer gpa.free(into);
     sh.put(p, prompt);
 
-    // `clear` while it runs erases what it printed before, and the pins then
-    // span rows that no longer hold it (ED 3 even moves one to the next page
-    // without calling it garbage), so only the erase itself tells.
+    // `clear` while it runs: what it printed after is its output, whole;
+    // what the clear erased was meant to go.
     var h = sh.send(p, node);
     sh.put(p, c ++ "before\r\n\x1b[H\x1b[2J\x1b[3Jafter\r\n" ++ d ++ prompt);
     var got = sh.answered(p, node, h, into);
-    try testing.expect(std.mem.startsWith(u8, got, "exit 0 cut\n"));
+    try testing.expectEqualStrings("exit 0\nafter\n", got);
 
 
     // A resize between its start and end reflows the pins with the text.