Share editor keymaps across TUI composer components (#38837)

## What changed

- Store the editor portion of `RuntimeKeymap` in an `Arc` so the chat composer
  and its embedded textarea use the same keymap snapshot.
- Initialize the textarea from the composer's default runtime keymap and keep
  custom binding updates shared between both components.
- Use copy-on-write when tests and binding resolution need to mutate editor
  bindings.

## Testing

- Verify that applying custom editor bindings shares the same keymap allocation
  with the composer and textarea.

GitOrigin-RevId: 7f3b0afd6cc784e23deee8819e5c2c37a3709c68

codex-rs/tui/src/bottom_pane/chat_composer.rs

@@ -322,6 +322,7 @@ use std::collections::HashSet;
 use std::collections::VecDeque;
 use std::ops::Range;
 use std::path::PathBuf;
+use std::sync::Arc;
 use std::time::Duration;
 use std::time::Instant;
 
@@ -515,7 +516,7 @@ pub(crate) struct ChatComposer {
     toggle_shortcuts_keys: Vec<KeyBinding>,
     history_search_previous_keys: Vec<KeyBinding>,
     history_search_next_keys: Vec<KeyBinding>,
-    editor_keymap: EditorKeymap,
+    editor_keymap: Arc<EditorKeymap>,
     vim_normal_keymap: VimNormalKeymap,
 }
 
@@ -707,6 +708,7 @@ impl ChatComposer {
             editor_keymap: default_editor_keymap,
             vim_normal_keymap: default_vim_normal_keymap,
         };
+        this.draft.textarea.set_keymap_bindings(&default_keymap);
         // Apply configuration via the setter to keep side-effects centralized.
         this.set_disable_paste_burst(disable_paste_burst);
         this
@@ -11222,8 +11224,10 @@ mod tests {
         assert!(matches!(result, InputResult::Submitted { .. }));
 
         let mut keymap = RuntimeKeymap::defaults();
-        keymap.editor.move_up = vec![key_hint::plain(KeyCode::F(2))];
+        Arc::make_mut(&mut keymap.editor).move_up = vec![key_hint::plain(KeyCode::F(2))];
         composer.set_keymap_bindings(&keymap);
+        assert!(Arc::ptr_eq(&composer.editor_keymap, &keymap.editor));
+        assert_eq!(Arc::strong_count(&keymap.editor), 3);
 
         let _ = composer.handle_key_event(KeyEvent::new(KeyCode::Up, KeyModifiers::NONE));
         assert!(composer.draft.textarea.is_empty());

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

@@ -39,6 +39,7 @@ use std::borrow::Cow;
 use std::cell::Ref;
 use std::cell::RefCell;
 use std::ops::Range;
+use std::sync::Arc;
 use unicode_segmentation::UnicodeSegmentation;
 
 mod vim;
@@ -127,7 +128,7 @@ pub(crate) struct TextArea {
     vim_enabled: bool,
     vim_mode: VimMode,
     vim_pending: VimPending,
-    editor_keymap: EditorKeymap,
+    editor_keymap: Arc<EditorKeymap>,
     vim_normal_keymap: VimNormalKeymap,
     vim_operator_keymap: VimOperatorKeymap,
     vim_text_object_keymap: VimTextObjectKeymap,
@@ -182,7 +183,7 @@ impl TextArea {
     /// the kill buffer, so callers can safely apply a live config update while
     /// preserving the current draft exactly as typed.
     pub fn set_keymap_bindings(&mut self, keymap: &RuntimeKeymap) {
-        self.editor_keymap = keymap.editor.clone();
+        self.editor_keymap = Arc::clone(&keymap.editor);
         self.vim_normal_keymap = keymap.vim_normal.clone();
         self.vim_operator_keymap = keymap.vim_operator.clone();
         self.vim_text_object_keymap = keymap.vim_text_object.clone();
@@ -2971,8 +2972,8 @@ mod tests {
         let mut t = ta_with("abc\ndef\nghi");
         t.set_cursor(/*pos*/ 5);
         let mut keymap = RuntimeKeymap::defaults().editor;
-        keymap.kill_line_start.clear();
-        keymap.kill_whole_line = vec![key_hint::ctrl(KeyCode::Char('u'))];
+        Arc::make_mut(&mut keymap).kill_line_start.clear();
+        Arc::make_mut(&mut keymap).kill_whole_line = vec![key_hint::ctrl(KeyCode::Char('u'))];
 
         t.input_with_keymap(
             KeyEvent::new(KeyCode::Char('u'), KeyModifiers::CONTROL),
@@ -3222,7 +3223,7 @@ mod tests {
         let mut t = ta_with("a\nb");
         t.set_cursor(/*pos*/ 2);
         let mut keymap = RuntimeKeymap::defaults().editor;
-        keymap.move_up.clear();
+        Arc::make_mut(&mut keymap).move_up.clear();
 
         t.input_with_keymap(
             KeyEvent::new(KeyCode::Char('\u{0010}'), KeyModifiers::NONE),
@@ -3237,8 +3238,8 @@ mod tests {
         let mut t = ta_with("a\nb");
         t.set_cursor(/*pos*/ 0);
         let mut keymap = RuntimeKeymap::defaults().editor;
-        keymap.move_up.clear();
-        keymap.move_down = vec![crate::key_hint::ctrl(KeyCode::Char('p'))];
+        Arc::make_mut(&mut keymap).move_up.clear();
+        Arc::make_mut(&mut keymap).move_down = vec![crate::key_hint::ctrl(KeyCode::Char('p'))];
 
         t.input_with_keymap(
             KeyEvent::new(KeyCode::Char('\u{0010}'), KeyModifiers::NONE),