Fix skill completion around bound mentions with suffixes (#33237)

## What changed

- Use `$`-mention boundary rules when deciding whether a skill mention followed by a suffix is already bound.
- Fall back to the editable skill token on the left when the token on the right is a bound mention such as `$bound/path` or `$bound.config`.

## Testing

- Add completion-target coverage for path and configuration suffixes.
- Add a composer snapshot covering the skill popup fallback for a bound path suffix.

GitOrigin-RevId: bf72237dfd56cfe6f590a3fab10c2bfe03b088a8

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

@@ -4064,6 +4064,12 @@ fn ends_plaintext_at_mention(bytes: &[u8], index: usize) -> bool {
     })
 }
 
+fn ends_plaintext_at_dollar_mention(bytes: &[u8], index: usize) -> bool {
+    bytes
+        .get(index)
+        .is_none_or(|byte| !is_mention_name_char(*byte))
+}
+
 fn starts_plaintext_at_mention(text: &str, index: usize) -> bool {
     if index == 0 {
         return true;
@@ -4114,9 +4120,7 @@ fn find_next_mention_token_range(text: &str, token: &str, from: usize) -> Option
         let ends_plaintext_mention = if sigil == b'@' {
             ends_plaintext_at_mention(bytes, end)
         } else {
-            bytes
-                .get(end)
-                .is_none_or(|byte| !is_mention_name_char(*byte))
+            ends_plaintext_at_dollar_mention(bytes, end)
         };
 
         if starts_plaintext_mention && ends_plaintext_mention {
@@ -6504,6 +6508,25 @@ mod tests {
         );
     }
 
+    #[test]
+    fn skill_popup_falls_back_from_bound_skill_with_path_suffix_snapshot() {
+        snapshot_composer_state(
+            "skill_popup_falls_back_from_bound_skill_with_path_suffix",
+            /*enhanced_keys_supported*/ false,
+            |composer| {
+                composer.set_skill_mentions(Some(vec![test_skill_metadata("left")]));
+                composer.set_text_content_with_mention_bindings(
+                    "$left  $bound/path".to_string(),
+                    Vec::new(),
+                    Vec::new(),
+                    vec![test_skill_binding("bound")],
+                );
+                composer.draft.textarea.set_cursor("$left  ".len());
+                composer.sync_popups();
+            },
+        );
+    }
+
     #[test]
     fn skill_popup_closes_after_trailing_space_snapshot() {
         snapshot_composer_state(

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

@@ -1,5 +1,6 @@
 //! Cursor-neighborhood resolution for sigil-prefixed composer completions.
 
+use super::ends_plaintext_at_dollar_mention;
 use super::ends_plaintext_at_mention;
 use super::is_mention_name_char;
 use crate::bottom_pane::textarea::TextArea;
@@ -268,7 +269,7 @@ pub(super) fn current_prefixed_token_range_with_dollar_predicate(
 /// Returns whether a token candidate's sigil and mention name are editable plaintext.
 ///
 /// A candidate is bound when either its entire range is atomic or its mention-name prefix is
-/// atomic and the remaining text begins with punctuation that terminates the mention.
+/// atomic and the remaining text begins with a terminator for that sigil.
 pub(super) fn prefixed_token_range_is_editable(
     textarea: &TextArea,
     prefix: char,
@@ -285,9 +286,14 @@ pub(super) fn prefixed_token_range_is_editable(
         .take_while(|byte| is_mention_name_char(**byte))
         .count();
     let mention_end = range.start + prefix.len_utf8() + name_len;
+    let ends_bound_mention = if prefix == '@' {
+        ends_plaintext_at_mention(textarea.text().as_bytes(), mention_end)
+    } else {
+        ends_plaintext_at_dollar_mention(textarea.text().as_bytes(), mention_end)
+    };
     !(name_len > 0
         && mention_end < range.end
-        && ends_plaintext_at_mention(textarea.text().as_bytes(), mention_end)
+        && ends_bound_mention
         && textarea
             .element_id_for_exact_range(range.start..mention_end)
             .is_some())

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

@@ -81,6 +81,24 @@ fn current_prefixed_token_falls_back_from_bound_at_token_on_right() {
     );
 }
 
+#[test]
+fn current_prefixed_token_falls_back_from_bound_dollar_suffix_on_right() {
+    for suffix in ["/path", ".config"] {
+        let text = format!("$left  $bound{suffix}");
+        let bound_start = text.find("$bound").expect("bound skill mention");
+        let bound_end = bound_start + "$bound".len();
+        let mut textarea = TextArea::new();
+        textarea.insert_str(&text);
+        textarea.add_element_range(bound_start..bound_end);
+        textarea.set_cursor("$left  ".len());
+
+        assert_eq!(
+            current_prefixed_token_range(&textarea, '$', /*allow_empty*/ true),
+            Some((0.."$left".len(), "left".to_string()))
+        );
+    }
+}
+
 #[test]
 fn current_prefixed_token_targets_later_segment_between_bound_elements() {
     let text = "$bound1$one$bound2$two$bound3";

codex-rs/tui/src/bottom_pane/snapshots/codex_tui__bottom_pane__chat_composer__tests__skill_popup_falls_back_from_bound_skill_with_path_suffix.snap

@@ -0,0 +1,13 @@
+---
+source: tui/src/bottom_pane/chat_composer.rs
+expression: terminal.backend()
+---
+"                                                                                                    "
+"› $left  $bound/path                                                                                "
+"                                                                                                    "
+"                                                                                                    "
+"                                                                                                    "
+"                                                                                                    "
+"  left  [Skill] Example skill used in tests.                                                        "
+"                                                                                                    "
+"  Press enter to insert or esc to close                                                             "