A pty/ctl winsize gives a terminal two rows at least, so it no longer reads busy for ever

A one-row pty loses the prompt's input mark as soon as the prompt wraps,
and busy detection never sees the shell idle again. Placement already
keeps a new terminal at tag + 2 rows; a winsize write now keeps the pty
at two. A pane dragged down to one body row keeps a one-row pty, since
a taller pty than the pane shows would put its cursor below the pane.

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

1 files changed, 3 insertions, 2 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 5c2486f4..9ba9c0e4 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -818,8 +818,9 @@ terminal (a REPL, `less`) takes none: write to `pty/data` for it --
 which is also when the third field of `pty/status` reads 1. `pty/status` is
 one line, three right-aligned fields and a newline: the pty's columns and
 rows, then busy (0 or 1). The size, and `pty/ctl`'s `winsize` read back, is
-what `winsize C R` last set, until the pane itself resizes and gives the pty
-its grid again. A line written
+what `winsize C R` last set (R of 1 is taken as 2: a one-row pty loses
+its prompt's mark and would read busy for ever), until the pane itself
+resizes and gives the pty its grid again. A line written
 before a new terminal's shell has drawn its first prompt is not busy: it
 waits for that prompt (a respawn meanwhile keeps it waiting for the new
 shell's) and is sent then, so the first command a script gives a fresh