tui: refine text area word separator handling (#5541)

## Summary
- replace the word part enum with a simple `is_word_separator` helper
- keep word-boundary logic aligned with the helper and punctuation-aware
behavior
- extend forward/backward deletion tests to cover whitespace around
separators

## Testing
- just fix -p codex-tui
- cargo test -p codex-tui


------
https://chatgpt.com/codex/tasks/task_i_68f91c71d838832ca2a3c4f0ec1b55d4

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

@@ -14,6 +14,12 @@ use textwrap::Options;
 use unicode_segmentation::UnicodeSegmentation;
 use unicode_width::UnicodeWidthStr;
 
+const WORD_SEPARATORS: &str = "`~!@#$%^&*()-=+[{]}\\|;:'\",.<>/?";
+
+fn is_word_separator(ch: char) -> bool {
+    WORD_SEPARATORS.contains(ch)
+}
+
 #[derive(Debug, Clone)]
 struct TextElement {
     range: Range<usize>,
@@ -841,21 +847,23 @@ impl TextArea {
 
     pub(crate) fn beginning_of_previous_word(&self) -> usize {
         let prefix = &self.text[..self.cursor_pos];
-        let Some((first_non_ws_idx, _)) = prefix
+        let Some((first_non_ws_idx, ch)) = prefix
             .char_indices()
             .rev()
             .find(|&(_, ch)| !ch.is_whitespace())
         else {
             return 0;
         };
-        let before = &prefix[..first_non_ws_idx];
-        let candidate = before
-            .char_indices()
-            .rev()
-            .find(|&(_, ch)| ch.is_whitespace())
-            .map(|(idx, ch)| idx + ch.len_utf8())
-            .unwrap_or(0);
-        self.adjust_pos_out_of_elements(candidate, true)
+        let is_separator = is_word_separator(ch);
+        let mut start = first_non_ws_idx;
+        for (idx, ch) in prefix[..first_non_ws_idx].char_indices().rev() {
+            if ch.is_whitespace() || is_word_separator(ch) != is_separator {
+                start = idx + ch.len_utf8();
+                break;
+            }
+            start = idx;
+        }
+        self.adjust_pos_out_of_elements(start, true)
     }
 
     pub(crate) fn end_of_next_word(&self) -> usize {
@@ -864,11 +872,19 @@ impl TextArea {
             return self.text.len();
         };
         let word_start = self.cursor_pos + first_non_ws;
-        let candidate = match self.text[word_start..].find(|c: char| c.is_whitespace()) {
-            Some(rel_idx) => word_start + rel_idx,
-            None => self.text.len(),
+        let mut iter = self.text[word_start..].char_indices();
+        let Some((_, first_ch)) = iter.next() else {
+            return word_start;
         };
-        self.adjust_pos_out_of_elements(candidate, false)
+        let is_separator = is_word_separator(first_ch);
+        let mut end = self.text.len();
+        for (idx, ch) in iter {
+            if ch.is_whitespace() || is_word_separator(ch) != is_separator {
+                end = word_start + idx;
+                break;
+            }
+        }
+        self.adjust_pos_out_of_elements(end, false)
     }
 
     fn adjust_pos_out_of_elements(&self, pos: usize, prefer_start: bool) -> usize {
@@ -1239,6 +1255,56 @@ mod tests {
         assert_eq!(t.cursor(), elem_range.start);
     }
 
+    #[test]
+    fn delete_backward_word_respects_word_separators() {
+        let mut t = ta_with("path/to/file");
+        t.set_cursor(t.text().len());
+        t.delete_backward_word();
+        assert_eq!(t.text(), "path/to/");
+        assert_eq!(t.cursor(), t.text().len());
+
+        t.delete_backward_word();
+        assert_eq!(t.text(), "path/to");
+        assert_eq!(t.cursor(), t.text().len());
+
+        let mut t = ta_with("foo/ ");
+        t.set_cursor(t.text().len());
+        t.delete_backward_word();
+        assert_eq!(t.text(), "foo");
+        assert_eq!(t.cursor(), 3);
+
+        let mut t = ta_with("foo /");
+        t.set_cursor(t.text().len());
+        t.delete_backward_word();
+        assert_eq!(t.text(), "foo ");
+        assert_eq!(t.cursor(), 4);
+    }
+
+    #[test]
+    fn delete_forward_word_respects_word_separators() {
+        let mut t = ta_with("path/to/file");
+        t.set_cursor(0);
+        t.delete_forward_word();
+        assert_eq!(t.text(), "/to/file");
+        assert_eq!(t.cursor(), 0);
+
+        t.delete_forward_word();
+        assert_eq!(t.text(), "to/file");
+        assert_eq!(t.cursor(), 0);
+
+        let mut t = ta_with("/ foo");
+        t.set_cursor(0);
+        t.delete_forward_word();
+        assert_eq!(t.text(), " foo");
+        assert_eq!(t.cursor(), 0);
+
+        let mut t = ta_with(" /foo");
+        t.set_cursor(0);
+        t.delete_forward_word();
+        assert_eq!(t.text(), "foo");
+        assert_eq!(t.cursor(), 0);
+    }
+
     #[test]
     fn yank_restores_last_kill() {
         let mut t = ta_with("hello");