## Why
Inside tmux, `Shift+Enter` can still reach Codex as a plain `Enter` even
when tmux has extended keys enabled. In `csi-u` tmux panes, Codex needs
to request `modifyOtherKeys` mode 2 so tmux moves the pane from `VT10x`
into extended-key mode and preserves the Shift modifier. Without that
extra request, composer `Shift+Enter` submits the draft instead of
inserting a newline.
Fixes#21699.
## What Changed
- Detect tmux sessions and read the active `extended-keys-format`,
preferring the pane-local value before falling back to the global
option.
- Request `modifyOtherKeys` mode 2 for tmux panes using `csi-u` extended
keys, and reset it when restoring keyboard reporting.
- Add unit coverage for tmux detection, the format gate, and the emitted
`modifyOtherKeys` escape sequence.
## How to Test
1. In tmux, configure:
```tmux
set-option -g extended-keys on
set-option -g extended-keys-format csi-u
```
2. Start Codex in a fresh tmux pane from this branch.
3. From another pane, confirm the Codex pane reports `mode=Ext 2`:
```bash
tmux list-panes -a -F '#{session_name}:#{window_index}.#{pane_index}
mode=#{pane_key_mode} cmd=#{pane_current_command}'
```
4. Type a draft in the composer and press `Shift+Enter`; confirm it
inserts a newline instead of submitting.
5. Also confirm plain `Enter` still submits as before.
Targeted tests:
- `cargo test -p codex-tui`
## Notes
- Manual verification used both real `Shift+Enter` in iTerm2/tmux and
`tmux send-keys ... S-Enter` to confirm the tmux pane changes from
`VT10x` to `Ext 2` and preserves newline behavior.
- On this checkout, the broader `codex-tui` test run currently reaches
unrelated existing failures in `status::tests::*` plus a later stack
overflow in
`tests::fork_last_filters_latest_session_by_cwd_unless_show_all`.
Felipe Coury
cac5354455