`sbx exec -it` does not propagate host terminal size to the sandbox PTY

[kkyr]

Summary

When attaching to a running sandbox with sbx exec -it, the PTY allocated inside the sandbox has no window size set. stty size returns 0 0 and the shell falls back to COLUMNS=80, causing prompts and TUIs to wrap incorrectly. Resizing the host terminal window after attach does not fix it — the new size is never propagated either. Passing -t/--tty explicitly (in addition to or instead of -it) makes no difference.

Reproduction

Using the base claude-code template (no customisation):

sbx run claude .
sbx exec -it claude-myproject bash

Inside the sandbox:

agent@claude-myproject:~/workspace$ stty size
0 0
agent@claude-myproject:~/workspace$ echo $COLUMNS
80

Now resize the iTerm window (drag a corner) and re-check — size is still wrong:

agent@claude-myproject:~/workspace$ stty size
0 0
agent@claude-myproject:~/workspace$ echo $COLUMNS
80

Same result with explicit --tty:

$ sbx exec -i --tty claude-myproject bash
agent@claude-myproject:~/workspace$ stty size
0 0

Expected behaviour

stty size inside the sandbox should report the host terminal's actual rows/cols, matching what stty size returns on the host. SIGWINCH should be forwarded when the host window is resized. This matches the behaviour of docker exec -it.

Actual behaviour

• Initial PTY size is 0 0 (kernel TIOCGWINSZ returns zeros).
• COLUMNS defaults to 80, LINES is unset.
• Host window resize is not propagated — size remains 0 0.
• Shells, prompts, and TUIs render as if on an 80-column terminal, causing visible wrapping (e.g. long prompts wrap mid-word).
• -t / --tty is accepted but has no effect on this behaviour — a PTY is allocated, but with zero dimensions.

Impact

• Interactive shells render incorrectly when the host terminal is wider/taller than 80x24.
• TUIs (Claude Code, vim, tmux, less, etc.) cannot lay out correctly.
• Workarounds require the user to manually run stty rows N cols N after every attach, which is fragile and doesn't handle subsequent resizes.

Environment

• sbx version: 0.26.1
• Host OS: macOS Tahoe 26.4