TUI: support zsh-style Ctrl+U as a configurable line-kill command

What variant of Codex are you using?

CLI / TUI

What feature would you like to see?

Add separate configurable editor commands for the two common Ctrl+U line-editing semantics:

• kill_line_start: kill from the beginning of the current line to the cursor
• kill_whole_line: kill the current line regardless of cursor position

The TUI currently binds Ctrl+U to kill_line_start, which matches Bash/Readline. That should remain configurable.

However, zsh's default emacs keymap binds ^U to kill-whole-line, and macOS uses zsh as its default shell. For the common macOS/zsh case, Ctrl+U should clear the current line by default, while Bash/Readline users should be able to restore the existing behavior through [tui.keymap.editor].

The simple approach is probably best:

• Add kill_whole_line.
• Default Ctrl+U to kill_whole_line, following zsh/macOS.
• Keep kill_line_start available as a configurable action for users who prefer Bash/Readline behavior.

This was also the primary papercut I had in mind when I originally started working on configurable keymaps: Ctrl+U is a small shortcut, but it is frequent enough that the wrong line-editing semantic becomes noticeable quickly.

Additional information

Relevant upstream behavior:

• zsh: kill-whole-line (^U) means "Kill the current line."

https://zsh.sourceforge.io/Doc/Release/Zsh-Line-Editor.html

• zsh also has backward-kill-line, meaning "Kill from the beginning of the line to the cursor position."

https://zsh.sourceforge.io/Doc/Release/Zsh-Line-Editor.html

• Bash/Readline: unix-line-discard (C-u) means "Kill backward from the cursor to the beginning of the current line."

https://www.gnu.org/software/bash/manual/html_node/Commands-For-Killing.html

• Apple documents zsh as the default shell on macOS:

https://support.apple.com/guide/terminal/change-the-default-shell-trml113/mac

This builds on #18593 by @fcoury-oai, which introduced configurable TUI keymaps.

A more user-adaptive version could choose the default from the user's shell configuration. For zsh, this is easy to inspect:

zsh -fc 'bindkey -e; bindkey "^U"'

On a stock zsh setup this reports:

"^U" kill-whole-line

For Bash/Readline, the current binding can be inspected with:

bash -ic 'bind -P | grep -E "^(unix-line-discard|backward-kill-line|kill-whole-line)"'

On a typical Bash/Readline setup this reports:

backward-kill-line can be found on "\C-x\C-?".
kill-whole-line is not bound to any keys
unix-line-discard can be found on "\C-u".

unix-line-discard corresponds to the existing Codex behavior: kill backward from the cursor to the beginning of the current line.

That said, shell probing is probably gold plating. It is more customer-friendly in principle, but it adds startup/runtime complexity and edge cases:

• probes may source user startup files
• probes may emit job-control warnings without a real TTY
• shell configs can be slow or have side effects
• $SHELL may not match the terminal/editor behavior the user expects inside Codex
• Windows behavior has not been considered here

A simple static default plus explicit configuration is likely easier to implement, document, test, and reason about. The adaptive heuristic can be treated as a future enhancement if maintainers think it is worth the extra complexity.

Existing implementation notes:

• The TUI currently has editor.kill_line_start and editor.kill_line_end.
• Ctrl+U currently defaults to editor.kill_line_start.
• The textarea handler maps that action to kill_to_beginning_of_line().

Relevant local files:

• codex-rs/tui/src/keymap.rs
• codex-rs/tui/src/bottom_pane/textarea.rs

Related issues and PRs:

• #18593 by @fcoury-oai: feat(tui): add configurable keymap support
• #3049: Configurable Hotkeys Support
• #10484: Add keyboard shortcut to clear entire multi-line input buffer
• #14582: Support ctrl+w / ctrl+u shortcuts for word or paragraph deletion