A detached session with no frontend takes its size from the root ctl

A --detach session nobody is attached to stayed 80x24, too small for an
agent's panes, and nothing could change it. The root ctl takes size <cols>
<rows> while no frontend is attached (refused while one owns the size), and
a client that attaches later still sets its own (the server compares with
the core's size). Documented with the 80x24 default.

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

1 files changed, 6 insertions, 0 deletions

diff --git a/test/fs.py b/test/fs.py
index 68da6c51..9336a53a 100644
--- a/test/fs.py
+++ b/test/fs.py
@@ -556,6 +556,12 @@ def cut_writes(binary):
             client.write(f'/pane/{serial}/ctl', block)
             body = client.read(f'/pane/{serial}/body').decode().splitlines()
             assert body == ['line %05d of the block' % i for i in range(2500)], (len(body), body[:2])
+            # A detached session with no frontend is 80x24 until told.
+            screen = client.screen()
+            assert (screen['cols'], screen['rows']) == (80, 24), (screen['cols'], screen['rows'])
+            client.write('/ctl', b'size 132 50\n')
+            screen = client.screen()
+            assert (screen['cols'], screen['rows']) == (132, 50), (screen['cols'], screen['rows'])
 
 
 def test(binary, quic=False):