Kill signals a command only while it runs, and only the pane it was meant for

A signal effect carried only a slot, so a Kill followed in the same step by
the pane's closing and its slot's reuse signalled the new pane's child; and a
command pane whose child had exited and been reaped kept a process group id
another process could take. The effect now carries the pane's serial, checked
as it is performed, and a host signals no command whose exit it has recorded;
the core already sends Kill only to a command it has not been told is done.

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

1 files changed, 2 insertions, 1 deletions

diff --git a/src/detached/server.zig b/src/detached/server.zig
index 202eafb3..5bf2384b 100644
--- a/src/detached/server.zig
+++ b/src/detached/server.zig
@@ -523,7 +523,8 @@ pub const Session = struct {
         const s = of(ctx);
         if (pane >= s.ptys.len) return;
         const pt = s.ptys[pane];
-        if (pt.fd < 0) return;
+        // A command whose exit is recorded is reaped: its group may be another's.
+        if (pt.fd < 0 or pt.cmd.exited) return;
         host_io.signalTty(pt.pid, pt.fd, sig);
     }