fix(tui): fix fallback phantom line and diff wrap at span boundaries

Two fixes:

1. highlight.rs: The fallback path used split('\n') which produces a
   phantom empty trailing element for inputs ending in '\n' (as
   pulldown-cmark emits for code blocks). Switch to lines() which
   handles trailing newlines correctly.

2. diff_render.rs: wrap_styled_spans only flushed the current line
   when col >= max_cols AND the current span had remaining content.
   This missed the case where one span ends exactly at max_cols and
   the next span has content — the next span's first character was
   appended to the already-full line. Remove the !remaining.is_empty()
   guard so the line flushes at span boundaries too.

codex-rs/tui/src/diff_render.rs

@@ -609,7 +609,9 @@ fn wrap_styled_spans(spans: &[RtSpan<'static>], max_cols: usize) -> Vec<Vec<RtSp
             remaining = rest;
 
             // If we exactly filled or exceeded the line, start a new one.
-            if col >= max_cols && !remaining.is_empty() {
+            // Do not gate on !remaining.is_empty() — the next span in the
+            // outer loop may still have content that must start on a fresh line.
+            if col >= max_cols {
                 result.push(std::mem::take(&mut current_line));
                 col = 0;
             }
@@ -997,6 +999,34 @@ mod tests {
         );
     }
 
+    #[test]
+    fn wrap_styled_spans_flushes_at_span_boundary() {
+        // When span A fills exactly to max_cols and span B follows, the line
+        // must be flushed before B starts. Otherwise B's first character lands
+        // on an already-full line, producing over-width output.
+        let style_a = Style::default().fg(Color::Red);
+        let style_b = Style::default().fg(Color::Blue);
+        let spans = vec![
+            RtSpan::styled("aaaa", style_a), // 4 cols, fills line exactly at max_cols=4
+            RtSpan::styled("bb", style_b),   // should start on a new line
+        ];
+        let result = wrap_styled_spans(&spans, 4);
+        assert_eq!(
+            result.len(),
+            2,
+            "span ending exactly at max_cols should flush before next span: {result:?}"
+        );
+        // First line should only contain the 'a' span.
+        let first_width: usize = result[0]
+            .iter()
+            .map(|s| s.content.chars().count())
+            .sum();
+        assert!(
+            first_width <= 4,
+            "first line should be at most 4 cols wide, got {first_width}"
+        );
+    }
+
     #[test]
     fn wrap_styled_spans_preserves_styles() {
         // Verify that styles survive split boundaries.

codex-rs/tui/src/render/highlight.rs

@@ -219,9 +219,13 @@ pub(crate) fn highlight_code_to_lines(code: &str, lang: &str) -> Vec<Line<'stati
         line_spans.into_iter().map(Line::from).collect()
     } else {
         // Fallback: plain text, one Line per source line.
-        code.split('\n')
-            .map(|l| Line::from(l.to_string()))
-            .collect()
+        // Use `lines()` instead of `split('\n')` to avoid a phantom trailing
+        // empty element when the input ends with '\n' (as pulldown-cmark emits).
+        let mut result: Vec<Line<'static>> = code.lines().map(|l| Line::from(l.to_string())).collect();
+        if result.is_empty() {
+            result.push(Line::from(String::new()));
+        }
+        result
     }
 }
 
@@ -291,6 +295,21 @@ mod tests {
         }
     }
 
+    #[test]
+    fn fallback_trailing_newline_no_phantom_line() {
+        // pulldown-cmark sends code block text ending with '\n'.
+        // The fallback path (unknown language) must not produce a phantom
+        // empty trailing line from that newline.
+        let code = "hello world\n";
+        let lines = highlight_code_to_lines(code, "xyzlang");
+        assert_eq!(
+            lines.len(),
+            1,
+            "trailing newline should not produce phantom blank line, got {lines:?}"
+        );
+        assert_eq!(reconstructed(&lines), "hello world");
+    }
+
     #[test]
     fn highlight_empty_string() {
         let lines = highlight_code_to_lines("", "rust");